Writers Cannot Do Math



"5/4 of people have trouble with fractions."

- (Anonymous)

You're watching a show or movie, or reading a book, when suddenly something numerical throws you for a slight curve—like a date, or a character's age. Your brow furrows. You start ticking things off on your fingers. What the hell? That wasn't right!

You have just discovered the fundamental truth: that your favorite author went to study literature because they failed irredeemably at high school math and went for something where they would never see a number ever again. This is a particular kind of continuity error that would be avoided if professional writers kept calculators at their desks. In their defense, some of these mistakes can be obscure and noticed only by die-hard fans. It can also come from multiple writers not checking with each other, or screwups in the timeline.

Compare Soap Opera Rapid Aging Syndrome, Not Allowed to Grow Up, and Longest Pregnancy Ever, where the writers can do math; they're just intentionally fudging it. See also Sci-Fi Writers Have No Sense of Scale and Artists Are Not Architects. Possibly the root cause of Everybody Hates Mathematics. Might even involve E=MC Hammer. You Fail Statistics Forever is a subtrope.

Anime and Manga

 * Ghost in the Shell: Stand Alone Complex: Major Motoko Kusanagi fought in World War IV in 2020. She must therefore have ended her five years of military training in 2010, and she must have been born at least 18 years earlier, in 1987 at the least. She was turned into a cyborg at age 5, thus in 1992... The writer probably just forgot to add all the cumulative time-periods together.
 * Saito's flashback episode in the second season indicates that Motoko served overseas in an unofficial capacity; officially, the Japanese Self-Defence Forces avoided combat during WWIV, except for recapturing Nemuro. Ishikawa says that nobody knows who she is, only that she's "a genius at combat" and that everyone calls her "the Major". How she came by that rank is somewhat up in the air.
 * Kusanagi didn't get her rank through normal process, but by getting hired by the Department of Defense as an irregular for her exceptional talent. It's heavily implied in the episode mentioned above that she is much more than she's letting on, only masquerading as a common soldier.
 * The eponymous Dragon Balls can't be used for one year after they grant a wish. However, only 8 months pass in between the first use in the series and the second one.
 * There's also the fact that if your family name is "Son", your age is going to get very confusing: Goku first claims to be 14, but nearly a year later claims that's because he thought 14 came after 11 so he's actually 12, yet according to the first point above, a year passed since he first gave his age, so he should be 13 (Then again... maybe his birthday is somewhere between May 7 and September 1. It's not like Goku knows his birthdate anyway. Or he was wrong the first time too). Gohan isn't much better: His age on the Buu saga is given as 16 when he should be chronologically 17, physically 18. And of course, there's the fact Goten is 7 when we first see him, but just under 7 years went in-between the Cell saga and its Time Skip to the Buu saga, so in order for him to be 7, he should have been born (not just conceived) during the Androids saga... where Chichi didn't even look pregnant. Figures.
 * Apparently, Toriyama has no idea how heavy a kilogram is either. It gets kinda ridiculous when Goku and Krillin, would could casually jump hundreds of feet in the air and outmuscle guys that travel by throwing and then jumping onto stone pillars cross-continent at super-sonic speeds, are barely able to lift fifty pound shoes. Apparently, an out of shape desk jockey in the real world would stand a good chance of winning the Tenkaichi Budokai.
 * Might be justified in that you wouldn't use much strength to lift a shoe if you didn't know beforehand that is much heavier than usual. Same goes the other way around when you think something is heavy but isn't actually, you use way more strength than needed, resulting in a "ripping it upwards" motion.
 * In Naruto, Jugo is 18, and doesn't look much younger in a flashback where Kimimaro convinced him to work for Orochimaru, who then developed the cursed seal from his body—yet Anko, who is six years older than Jugo, had the curse mark put on her when she was still a young teenager (though that scene was only in the anime, it still seems Jugo became Orochimaru's test subject much later in life than he would need to be). A comment by Zetsu in Part II stated that Orochimaru left Akatsuki after trying to take Itachi's body 10 years ago, meaning Itachi was in Akatsuki two years before the Uchiha Massacre, not to mention being 11 at the time.
 * It gets worse. Supposedly Orochimaru conducted the Mokuton experiments on infants, or at least young children, but Yamato's age is given as twenty six in the second part of the manga, and Orochimaru didn't know that he had survived because he was forced to flee the village. This would mean that he left Konoha at least twenty years before the start of the manga. However, Kabuto's flashbacks in the recent manga indicate that Orochimaru was still a part of Root as late as seven years before the start of the manga. It's impossible to make any kind of coherent timeline out of these events.
 * When there are some counting errors in the art: When he starts going into his tailed beasts forms, one forms is shown with eight tails, but someone on the sidelines says there are seven and when he's reduced to one he says he lost six of them. When he takes another form we see seven tails in one panel, but eight in the next two. These were both corrected in the volume version: the first form had one of the tails erased and the second had a tail added.
 * Similar to the Harry Potter example below, Konoha's ninja population seems iffy. During the 4th Shinobi World War, we are told that the Allied Forces are comprised of some 80,000 ninja and samurai. Konoha supposedly has the largest population of any ninja village, but even if each village and the samurai contribute the same amount of soldiers, Konoha would still have to have at least 13,000 nin. Though we only ever see the same 50 Konoha ninja, this is still possible. It's just that, according to Kabuto, there were 87 Konoha genin in the Chunin exam and, even accounting for genin who couldn't or didn't want to particpate, that seems like an incredibly small ratio. Not to mention that at the time of the chunin exams, the Rookie 9 are apparently the only new genin in Konoha. The exams are done twice a year, so that would mean that Konoha graduates at most 18 ninja a year.
 * In an episode of Digimon Savers, Marcus Damon has a flashback of one of his birthdays, in which he fell and tore open his head and his father ran him to the hospital. In the original Japanese, which birthday this was isn't stated, but in the dub, he had just turned six. How can this be possible if Marcus is 14, and his father has been missing for 10 years?
 * In an episode of Doraemon, Doraemon gives Nobita a special plate of cookies that double every 5 minutes (so if he starts with one, after 15 minutes he will have 8 cookies), warning him that he must eat every single one of them quickly. In the end, he throws away one and returns after a while to find a big pile of cookies. Unable to eat them all, they choose to throw them to space, and later Doraemon tells Nobita that a whole galaxy made of cookies has formed. In reality, if they doubled every five minutes, in less than 15 hours the mass of the cookies would be many orders of magnitude larger than that of the whole universe. Oops.
 * Yu-Gi-Oh! examples:
 * In Yugi's duel with Weevil Underwood at the start of the Duelist Kingdom arc, Yugi plays a card that raises Summoned Skull's attack power by 30%. 30% of 2500 is 750, but the attack counter goes up by 1000 points. This was corrected in the dub version.
 * In Battle City, Yugi defeats Strings - who is using Slifer the Sky Dragon, a God Card that has 1,000 ATK per card in the user's hand - by setting up a continuous loop that forces Strings to draw cards until he is decked out. Slifer, as a result, gains ATK continually, despite Strings being unable to order an attack. Its ATK score shown before Strings is decked out is 23,000. However, assuming Strings was using the minimum 40-card deck, Slifer's maximum here should have been 28,000; he had had 5 cards on his field and 7 in his Graveyard that he had already played, giving him a hand size of 28 when he was decked out.
 * In Yu-Gi-Oh! GX, in the first duel against the Supreme King, there is one sequence where the King takes damage. Except that the damage he actually took is about 100 less than the damage he should have taken. This is Egregious because had he taken that extra 100 damage, he would have wound up losing the duel.
 * The same scoring error appears in Yugi's duel against mind-controlled Bandit Keith. Then again, since Keith is quite explicitly cheating, Yugi deserves a few extra lifepoints to give him a fighting chance.
 * In Mitsudomoe the class is playing anything goes chairs at the start of the manga and anime. The rules are simple - there are enough chairs in a circle to seat all but one member of the class. Whoever is left out has to call out a criteria, anyone who matches that criteria has to leave their chair and find a new one. After their initial rush for seats it appears that everyone in the class is seated. The teacher is confused at first thinking that there should be exactly one seat less than there are people - it turns out the eldest daughter Mitsuba is sitting on a classmate who is on all fours. Catch is, if she is sitting on a classmate instead of chair then there are two people not in chairs and thus an empty chair somewhere which would have been obvious.
 * In a Black Butler OVA, Thomas Wallis was born in 1775, and his scheduled death date was 1779. This would be fine if the boy didn't appear to be in his early 20's at the time of death.
 * In a season two episode of Mega Man NT Warrior, Numberman is tied to a giant bowling pin by Bowlman, who starts knocking pins down. Numberman calculates the odds of his pin being hit as 10%, 20%, 30%, etc as the pins fall. Even if one assumes that Bowlman will only hit one pin with each ball, and that the odds of any given pin being his is identical (Which is most definitely not a sensible assumption when bowling), the odds of Numberman being hit would have been 1 in 10, followed by 1 in 9, 1 in 8, etc, or 10%, 11.1%, 12.5%, 13.3%, etc.
 * In episode 5 of season 2 of Fate/Zero, a set of shelves loaded with enchanted flowers features prominently and...confusingly. In one shot of them (over shoulder), 5 rows of shelves are visible, but 30 seconds later the entire shelving unit is shown from another angle as Shirley runs over to them, and there are only 4 rows! The 4-rows version remains consistent for the rest of the show, but there's another problem...
 * ...between those two shots, a flower which is supposedly #100 is added to the last open slot on the shelves. In the very first shot containing the shelves (the bird's-eye view of the house), the top of the shelves is obscured and there appear to be 30-40 on the floor, so 100 is a believable number. After this shot, most of the floor flowers vanish, and with only 4 rows of shelves the math doesn't work out. There are only 48 flowers on the shelves and not more than 10-15 on the floor next to them, which is only about 70 flowers at the most. It's possible some of them are stored elsewhere, but either way, a lot of flowers mysteriously disappear between shots.

Comic Books

 * The Flash saves a Korean city from a nuclear blast by carrying each citizen to a safe distance of 35 miles away at "a hair's breadth short of the speed of light" in .00001 of a microsecond (or 1/100 billionth of a regular second). Except if you do the math, getting that many people (stated to be one half million) so far away that fast would actually require him to have to be traveling thirteen trillion times the speed of light. The problem is the time period being so ridiculous short (way shorter than it needs to be, even to escape from a nuclear blast) that even the speed of light isn't fast enough to go 3mm one time.
 * Leaving aside the little problem of inertia: his rescuees should be pulp, unless Flash's apparent immunity to G-forces extends to folks he's carrying.
 * Justice Society of America member Stargirl is explicitly stated to be sixteen years old early in the book's run, and is still that old later on. Then, 52 and One Year Later happen as a Time Skip thing. Then she celebrates her birthday. By all rights, she is now eighteen.
 * It seems the policy of DC Comics is to no longer mention ages for any of their teenage characters. Supergirl also went through a recent birthday that, logically, would have been her 18th, but her age was not mentioned and she seems to be kept in the same "late teens" range as Stargirl and Wonder Girl. Possibly subverted with Tim Drake (Red Robin), since while his age hasn't been mentioned since his 16th birthday (pre-War Games), he seems to have aged into young adulthood and is traveling around the world without a guardian.
 * Jean Grey's age in Ultimate X-Men was originally given in promotional materials as 15. When one of the book's writers began a plot with her relationship with Wolverine, her age became relevant and had to be retconned to 18.
 * More a case of "Writers don't get units", but in an issue of the Second Coming crossover in X-Men, the bad guys - a loose coalition of anti-mutant paramilitants - list off their armies, starting with "50 bases with a hundred men each" (so 5000 men, sizeable), "numbers in the thousands" (still large) and finally "40 armoured divisions." The latter would exceed 400,000 men, and thousands of tanks - a completely ridiculous number for the organization in question. For comparison, the United Kingdom's entire armed forces currently consists of six divisions, ONE of which is an armored division, 40 armoured divisions would be larger than the entire Indian army and only slightly smaller than the Chinese one. 40 divisions would also be about twice the strength of the current U.S. Army.
 * Elf Quest's timeline is by now properly beyond all help, because of too many authors Running the Asylum. Particularly Mantricker's timeline, the era between Goodtree's rule and Bearclaw becoming chief, makes no sense whatsoever anymore.
 * Superman demonstrates "Super Math" in this comic panel, but the writer is off by a factor of ten.
 * The Tintin book Destination Moon shows, at one point, a calendar page reading Thursday 13 May. The rocket launch takes place on Tuesday 3 June. These dates are exactly three weeks apart and therefore the same day of the week in any year.
 * Marvel's Canon Discontinuity miniseries Trouble purported to reveal that Aunt May was actually Peter Parker's biological mother. Problem is, writer Mark Millar presents Peter as the result of a teen pregnancy. Which would make May less than thirty-five when Peter gets his powers, instead of being in her sixties (616 Universe) or fifties (Ultimate Universe). Maybe baby Peter spent thirty years as a Human Popsicle?

Fan Works

 * In Light and Dark - The Adventures of Dark Yagami, a "sevensome" goes on between six people: 1)Sayu, 2)Misa, 3)Dark, 4)Light, 5)Light's mom and 6)the girl from the bus.

Film
"Aaron: ...sometimes the product of two negative integers is a positive number. Cady: Yeah, like negative four and negative six."
 * The Heavenly Kid has the title character die in what is clearly the late '50s or early '60s. He's then brought up to The Present Day (1985) and discovers he sired a son, who's now in High School. Apparently, his son had to repeat a few years.
 * It's a Wonderful Life: Clarence says that, in the altered reality, Harry Bailey died at the age of nine, but the gravestone right in front of him as he says the line reads "1911 - 1919", making him either 7 or 8.
 * Pi: Mathematician/genius Max Cohen tells the Kaballists that he can't just tell them the 216 digit MacGuffin number because "You've already written down every 216 digit number and intoned them all and what has it gotten you?" To do so would, of course, take even a large group of researchers, such as the entire population of the Earth, significantly longer than the age of the Universe to do, and inconceivably more ink than there is mass in the universe - indeed, even if only one electron were needed to write down each number, 10^100 universes would be much too small. Any mathematician should be well aware of this. Of course, Max and reality don't always see eye to eye, especially considering he's having a schizophrenic breakdown at that point.
 * Also, the Kaballists are after it because they're looking for a 216 character word. Since this would be in Hebrew, which has 22 characters, they actually want a number with roughly 290 digits. So Max probably doesn't even have what they want.
 * Played with in Stranger Than Fiction: Harold is asked a complicated math question and can't think because of the narrator in his head. She tells him an answer, which he promptly says out loud. She then says that answer was wrong and gives another one, causing Harold to apologize and switch to the new answer. The first answer was the right one. This was intentional on the part of the writer.
 * This was mostly averted in the Back to The Future films. Every time a date is mentioned in the trilogy, the day of the week paired with it is always accurate. For example, November 12, 1955 really was a Saturday. The characters' ages also add up correctly, though the filmmakers made the math as easy as possible. Everyone in Marty's parents' generation (aside from Biff, who was one year older than George and Lorraine) was seventeen in 1955, forty-seven in 1985 and seventy-seven in 2015. And everyone in Marty's generation is seventeen in 1985 and forty-seven in 2015. However, according to the DVD features, The Honeymooners episode that Lorraine's family watched did NOT air on November 5, 1955. However, all the sports scores listed on specific dates in the sequel WERE the actual scores on those dates.
 * The infamous "The sum of the square roots of any two sides of an isosceles triangle is equal to the square root of the remaining side" mangling of the Pythagorean theorem, as said by the scarecrow after he got his brain in The Wizard of Oz. It should be, "The square of the hypoteneuse of a right triangle is equal to the sum of the squares of the other two sides."
 * Lampshaded in The Simpsons, where Homer says the exact same line in an attempt to sound smart after putting on Henry Kissenger's lost glasses, and a person in the bathroom yells out, "That's a right triangle, you idiot!" Homer: "D'OH!"
 * This may be intentional since, in the film, Scarecrow didn't get a brain. He got an honorary degree instead, complete with diploma, making him a person everyone regards as intelligent without necessarily possessing the required intelligence. The false statement lampshades how he can sound impressively smart while not being so.
 * Transformers: Revenge of The Fallen: 5 Decepticons go into the water to revive Megatron. When they get down there, they kill one for parts and revive Megatron. When they go up, a sub commander says that six are going up.
 * Since Scalpel was hiding inside one of the Constructicons when they went down, the commander was technically right. On the other hand, Scalpel is less than a foot tall. If the actual intention of the line was that the sonar detected five 60-foot robots going down and five 60-foot robots plus one ten-inch something coming up and that was serious cause for alarm, we can probably agree that's even stupider.
 * The script originally called for Blackout to be revived here as well (Hence him being at the forest battle... Except no, now he's Grindor!) Just no one bothered to change other bits of the script...
 * That probably was Blackout (they never gave a name in the film) in the forest, and he was just revived offscreen. Or the dude they ripped apart for spare parts got better on the spot.
 * In Forrest Gump, Forrest claims that Jenny died on a Saturday, and yet her gravestone says 22 March 1982, which was a Monday. Odd that Robert Zemeckis would make that mistake, considering he so thoroughly avoided it in the Back to The Future films, as noted above.
 * Cube: Leaven, a mathematician, spends more than half a second to determine if 645 and 372 are prime numbers. Any multi-digit number ending in 2 or 5 is not prime. Even if they didn't, it is not an "astronomical" calculation to factor a number less than 1000. You don't have to be a savant to do this in your head.
 * One scene in St Trinian's had Stephen Fry award points to a team for concluding that the volume of a sphere is πr^3. A fourteen-year-old could probably tell you that it's actually (4πr^3)/3.
 * Clearly the writer extrapolated from πr^2 giving the area of a circle. Although he claimed in his biography to enjoy studying maths under his father, Stephen Fry is much better versed in humanities than mathematics, otherwise he might have spotted this one.
 * In Never Been Kissed, the geeky kids sell pies at a bake sale, with a sign proclaiming "pi = 3.1457" followed by some more digits to make it look sufficiently nerdy. Only the first three digits are correct; the rest is nonsense.
 * This could be just pure authorial laziness. The simple act of hitting the π key on an eight-digit calculator will bring up the number 3.1415927.
 * Also, Josie's initial high school tenure is played against the backdrop of The Eighties, when, based on the dates and ages mentioned, they actually should have occurred in 1992, 1991 at the earliest. Instead of the soundtrack you'd expect to hear for a sequence set during the heyday of Grunge, Josie and her classmates are always shown cavorting around to Cyndi Lauper and The Smiths, music more apropos of what should have been their grade school years.
 * Very poor aging math in Up. Carl is mentioned via Word of God to be 78; so he would be born roughly 1930. This means that he met Ellie was in 1938-1942; whilst the cars would suggest more 1926. Also note he sees a reel of Charles Muntz, who is about 20 years older than him. When he's middle aged, 28 AT LEAST, during the married life sequence, there's a bit with an imitation 40' Ford, in ~1958+ . Maybe Up is set some years in the past? And when he meets Charles Muntz, he's about his age and active - while he should be 98 years old...
 * Terminator's Judgment Day was to occur August 1997. John Connor was born February 1985. Terminator 2 shows him at the age of ten being attacked by the Terminator. Terminator 3 then Retcons his T2 age to thirteen. The problem is that he wouldn't turn thirteen for six months after the bombs supposedly fell in Kyle Reese's timeline.
 * Sarah's age is also an issue. In the first movie (set in 1984), she is presumably 18 (or possibly 19), and in the second movie (set in 1995), Dr. Silberman explicitly refers to her as 29 years old. But in T3, the gravesite indicates that she was born in 1959 - which is incompatible with the previous movies.
 * Fridge Brilliance! Sarah has been consistently lying about her age and birthdate after the first movie to make herself harder to track by skynet.
 * Alex O'Connell's age in The Mummy Returns. He's eight in 1933, so he'd have to have been born in 1925. Rick and Evy met in 1926. Neat trick, Alex.
 * In Roxanne, a modern-day remake of Cyrano De Bergerac, C. D. Bales agrees to make twenty jokes about his own nose, in what qualifies as a Crowning Moment of both Awesome and Funny. Apparently he and the rest of the bar patrons lost track. He actually did twenty-five jokes.
 * Personal theory: The writers had the patrons lose track on purpose. This way, when they edited the film for network TV, they could leave out the filthier jokes and still have a full twenty.
 * In the James Bond movie GoldenEye, the Big Bad  Seeing as how he's roughly the same age as Bond (somewhere from mid-30's to early 40's), he's too young to exist before then.
 * This was averted in the 2010 Wii remake of the original game of the movie: since the game is set in the present day, said Big Bad's motivation is changed to
 * In Mean Girls Cady is supposed to be a mathematical genius, so the writers used really overcomplicated explanations of how to do simple things like working out percentages, but seem to be okay in terms of accuracy. However, Cady intentionally makes mistakes to help her crush/tutor feel smart, leading to moments like this little exchange:


 * Seriously guys, the product of any two negative numbers is positive. God! You're so stupid.
 * In Night at the Museum: Battle of the Smithsonian, an Einstein bobblehead (supposedly as smart as the real Einstein) claims that pi is exactly 3.1415.
 * Kellys Heroes, a World War Two film, features a bank heist of 14,000 bars of Nazi Gold supposedly worth $16 million in 1944 dollars. No matter how you do the math, these figures cannot be reconciled with (a) the number of shares, (b) the observed weight and size of the bars, (c) the number of boxes. See the Fridge Logic page for details.
 * In The Dark Knight, at the end of the movie, Com. Gordon mentions that there are five deads bodies, two of which, were cops in regards to 's rampage. The killer was shown killing three people on screen and while there is a quick blink-and-you'll miss it shot of the killer grabbing one of Mornoni's men, assuming that the henchman was killed and not knocked out, that is still only four bodies. Aside from that, there was only one cop killed in that rampage, not two.
 * Gordon might have been assuming, given the facts he knew at the time, that the cop who was spared was actually killed (technically "missing in action" at that point). Christopher Nolan has reported that he'll explain that line at some point. Just not yet.
 * In X Men First Class there is a scene with Charles Xavier as a child which is set in 1944, while the bulk of the story takes place in 1962. However, the two actors are credited as playing "Charles Xavier: 12 years" and "Charles Xavier: 24 years."
 * Entrapment: Catherine Zeta-Jones needs ten extra seconds after midnight in order to use a computer program to steal billions of dollars from an international bank. After 11pm, a device she set up "steals" 1/10th of a second every minute until midnight, which will total 10 seconds by midnight. But that only equals 6 seconds, which is 4 shy of the required 10.

Literature
"Master: Yeah. Look, say you've got a hundred feet of fence to put up and you need to put a post every ten feet. How many posts do you need? Pupil: Ten, of course. Master: Nope. Eleven. Unless you string your fence in a circle. If you put the posts in a closed figure, you only need nine because you start and end on the same post. Pupil: And how am I to know such things?"
 * An Older Than Print example: In the Nibelungenlied, Siegfried and Kriemhild have a son who is born in the tenth year of their marriage. Shortly after that Siegfried is murdered by Hagen. Kriemhild then spends three and a half years mourning for him before accepting Etzel's proposal. Twelve years into her second marriage she invites her brothers for her party of revenge. So the second half of the epic takes part more than 25 years after the first part (from Siegfried's arrival at Worms to his and Kriemhild's wedding), but characters don't age accordingly. Giselher, the youngest of the brothers is still referred to as "the child", while Siegfried's son disappears from the narrative without explanation after Siegfried's death even though he should be about sixteen, grown up by medieval standards, by the end. Hagen's feat of single-handedly sinking Siegfried's gigantic hoard (more than 1000 wagonloads) in the Rhine is, shall we say, highly improbable, as was his later feat of ferrying the Burgundian train (10,000 men) across the Danube in one day with just one boat.
 * In Rainbow Six, we are told a group of terrorist is composed of 15 people. However... There's three trucks with three terrorists in each (9). There's five in the hospital (14), the boss (15), his second (16). Furthermore, we are told only six of the fifteen (well, sixteen) survived. Which is not true: four of the hospital group surrendered, the boss was captured alive, as was one of the truck drivers. Furthermore, an entire truck was flashbanged, and all three of it's occupants were captured, which would make for nine prisoners, not six.
 * The Harry Potter series has many examples, generally called "oh dear, maths" moments after the World Book Day interview, where J. K. Rowling admitted she's not good at mathematics:
 * In Quidditch Through the Ages, it says that the first ever World Cup was in 1473, with a match being held every four years. Yet the World Cup in Goblet of Fire, set in 1994, is number 422. There can't be 422 world cups in 521 years with a four-year gap in between. Calculated correctly, the World Cup that the Weasleys, Hermione, and Harry attend is actually the 131st, and it would have been held in 1993 if not for having been banned for significantly more than four years before the 1994 event.
 * If you take every date in the books seriously, you're lead to the conclusion that Andromeda Tonks could have been at most twenty when her daughter was born. Not too unusual, but to make her that young you have to assume that Andromeda and Bellatrix are twins, and both of them quite a bit younger than Narcissa...
 * Then you have Tonks and Teddy. There is time for the pregnancy, but it's really squeezed in there. Tonks was either pregnant when she got married, or very soon after, and behaves as though she's aware of the pregnancy as soon as it would have started.
 * Seeing as how she's a shapeshifter, that one makes sense, in that she'd know rather quickly if there were parts of her body she couldn't change.
 * The dates in the story must have been undecided-on until late in the game, because Dudley has a PlayStation in the fourth book (which would have taken place in 1994, before it was released in Japan, let alone Europe) - Rowling admitted she Did Not Do the Research with that one.
 * The most commonly noted example: It's implied (but never explicitly stated) that students are divided more or less evenly into houses. There are 5 Gryffindor boys in Harry's year, as well as 5 named Slytherin boys. That suggests there are roughly 40 students in Harry's year—and J.K. Rowling even has a notebook listing them. If we assume this is a normal year... we are led to the conclusion that there are no more than 280 students attending Hogwarts. J.K. Rowling has stated that she had just been so proud to have 40 rounded-out characters that she didn't do the math. She envisions Hogwarts to have somewhere around 750 students; and there is a line about "200 students in green" during one of the Quidditch matches, supporting the view of a larger student body.
 * But there's a more severe problem with the "large Hogwarts" view: there are fourteen subjects, just one teacher for each (so says Word of God, with the exception of Divination after the events of the fifth book) and on several occasions it's mentioned that there are 20 students in each lesson. If there were 750 students, either the day would be two-thirds free periods or the teachers would all have to be using Time Turners to teach three classes at once.
 * The math is still weird even if you assume 280 and that each year has two classes, each with two Houses in it, which is what the book implies. And there are seven years, although it's likely that the last two years only have one class for each subject as students drop subjects and even drop out. This means each teacher teaching the five required-for-the-first-five-year subjects is handling 12 different classes of 20 students each, for a total of 240 students.
 * Sounds good, but then it gets weird, as there are five slots a day for classes, so only 25 slots each week. Assuming the teachers get at least one free period a week, that means that they can only have two classes for each grade a week. Which sounds workable, and even allows the 'Doubles' class periods which show up, but then blows up in your face when you realize five required classes, of two hours each is...ten hours. First years have an added flying class, although that can't add more than a few hours a week, and third years and beyond have two electives (Or more if you're Hermione), but second years apparently have a ten hour workweek!
 * Related to this is the fact that Umbridge could not possibly have had time to inspect so many lessons as well as teaching her own.
 * And speaking of the schedule, as class is an hour long, and they start a 9:00, there probably shouldn't be two classes, a break, and another class before lunch, as the break seems to be at least thirty minutes, possibly an entire hour, and logically pushes lunch to at least 12:30. Then there are a break right after lunch, two more classes, and then free time until dinner. It's not mathematically impossible, but it is a strange schedule, to say the least. Why not have one of those classes after lunch? And why have a break right after lunch? Of course, as statistically they only have enough teachers to be in class two or three of the five slots each day, it really doesn't matter much.
 * In short, all the more time to Wake Up, Go to School, Save the World. Or snog some other kid while getting lost in the shifting hallways.
 * Additionally, there's the matter of Ron's older brothers, Bill and Charlie. Originally, JK Rowling stated that Charlie was two years older than Percy, and Bill was two years older than Charlie. However, Percy is in fifth year in "Philosopher's Stone", meaning that Charlie should have either been in seventh year, or just graduated. She later corrected this to Charlie being three years older than Percy - which is still impossible, as the Gryffindor Quidditch team has allegedly not won the Cup since Charlie stopped playing for them, and the period of time since Gryffindor has won is given in "Prisoner of Azkaban" as having been seven years, coming up on eight (or four years before the start of the first book). Charlie would have had to have left the team in his third, or possibly fourth, year for his age at three years older than Percy to be plausible - unlikely, since he was Quidditch Captain and "could have played for England".
 * Also, in Chamber of Secrets, Ginny mentions wanting to attend Hogwarts since Bill came. Bill went to Hogwarts a year after Ginny was born (and that's without the earlier dates from the paragraph above). So Ginny wanted to go to Hogwarts since she was one? Or before she was born? It was also unnecessary because she has 4 older brothers she would've been able to bond with before they went off to Hogwarts. (Of course, she wasn't exactly thinking clearly at the moment.)
 * Then, of course, there is the number of wizards and witches in Great Britain, which Rowling puts at around 3,000. That's all fine and good . . . until you start wondering how they support multiple professional, regional Quidditch teams, among other things, with a population that small.
 * If there are 750 students and 3000 wizards, then a quarter of the entire wizarding population is attending Hogwarts at any one time. Which means that at least a quarter of the population is between 11 and 18. "You Fail Demography Forever"?
 * If there are only 3000 wizards in all of Great Britain, how do they have such a sprawling governmental bureaucracy? There wouldn't be nearly enough tax revenue for what we see, not even close. Unless the entire Ministry isn't getting paid...Also makes you wonder how the economy works. How do the Malfoys have so much money? We never see their business, and there really aren't enough consumers for anyone to get very wealthy. It could be mostly inheritance, but that just pushes the issue back a few generations.
 * Averted hard in The Lord of the Rings. Tolkien's meticulous attention to detail is seen not only in the appendix which deals with dates and calendars (showing knowledge of astronomy) but also in the body of the text where whenever the moon's phase is mentioned, it is consistent with every other mention. Dates also remain consistent, remarkable for a text that was written sporadically over a period of more than a decade. This is even more impressive when you consider the larger mythos, which is not only consistent within itself but can be made to match to actual history and legend (in terms of major events, other mythologies, the supposed sinking of Atlantis/Númenor and matching Gondor with the Megalithic Culture in Europe, as well as New Age ideas) with surprising accuracy. This is a case of Fridge Brilliance, as Tolkien never even hints at the vast majority of these little touches, nor does he ever Show His Work in the same way he does with his languages and it took independent readers to notice.
 * There's one instance in the entire legendarium where he gets a date wrong, but it doesn't count because it's in The Hobbit. (When he first wrote it, Tolkien thought of The Hobbit as taking place in a separate continuity from the legends of the Silmarils and Numenor. Only while working on The Lord of the Rings did he decide to Retcon The Hobbit into the Middle Earth legendarium.)
 * The Giver has ridiculously strict population control methods doomed to fail. Even with a completely cooperative populace, it will still fail because of math.
 * Each family unit is allowed a maximum of 2 children, the same number of children are born each year and they are all assigned to a family unit. Not all adults have children, and not all family units have the maximum of 2 children.
 * Birth mothers, the only job that allows giving birth, are only allowed to have 3 children each before they become laborers. This would require that at least 2/3 of all women become birth mothers to maintain a stable population, but this doesn't happen at the beginning of the book as the administration is handing out jobs to graduates. Of course, it's not as if this would be a job for life.
 * There's a children's book titled Brog the Stoop in which the race of Stoops are restricted to one child per family. The author really should have noticed that this would lead them to die out very quickly, each generation being at most half the size of the previous one.
 * A plot point in The Power of Un, where the Time Travel MacGuffin has to accept its figures as the number of minutes to be rewound—so, of course, the hero gets his math wrong and accidentally goes back to the beginning of the previous day. However, the author gets his math wrong twice; first of all, the time he ends up at is not consistent with his answer, and, when Rainy is asked to provide the correct answer (much to her bemusement, since to her, it's just a random, spur-of-the-moment multiplication problem), she gets a result which is still incorrect.
 * In The Legend of Rah and the Muggles, Stubby aged from five to fifteen over the course of twenty-two years.
 * Timeline difficulties are very common in Tortall. Some fans have spent a lot of effort trying to make sense of it all.
 * Rick Cook's fantasy novel The Wizardry Compiled contains this fantastic explanation of the concept of "fencepost error" by a supposedly guru-level programmer:

This was corrected in later editions.
 * In case you don't get it, the error is that he says nine posts for a closed figure. You would really need ten. To visualize this, imagine a fence laid out in a triangle, with three rails and three posts.
 * A Series of Unfortunate Events has quite a few ages and timelines that don't seem to fit together. Then again, the Backstory is laden with unreliable information and all sorts of things which make very little sense.
 * In the Henry Reed series of books, it may look this way. Aging naturally for the first four books, then rebooting back for the fifth written much later. In reality it was probably a case of Comic Book Time. Who cares about sixteen-year-old protagonists when you can contradict your own rules and make him thirteen again?
 * Stephen Erikson has some issues with timelines in Malazan Book of the Fallen. While, at first glance, everything seems to function fairly well, many dedicated fans of the series trying to put together a timeline of events quickly realized that for the books' narrative to make sense, some events would have had to happen almost a decade after the time they were stated to take place, while others would have to happen before events that chronologically occurred later and, in one case, a particularly important event would have to occur before the events that lead to it.
 * According to the last couple of volumes of The Dark Tower, in which Stephen King uses himself as a character, Stephen King was 22 in 1977, despite having been born in 1947. Possibly justifiable in that parallel worlds are of extreme significance in the story, and it's quite possible that the Stephen King of "Keystone Earth" was born in 1955.
 * Discworld intentionally avoided a precise timeline for a long while, meaning that the course of events in the series looked occasionally like a horrible mess, and readers have gone to elaborate lengths to construct a working timeline. Terry Pratchett eventually paid more attention to the timeline, and shrugged off inconsistencies as alternate pasts via the "Trousers of Time". Eventually, he got right down to handwaving the inconsistencies in Thief of Time, which established that time has been "broken" before and fixing it all together didn't go as well as hoped.
 * The three Jules Verne novels 20,000 Leagues under the Sea (set in 1866-1867), Captain Grant's Children (1862-1864) and The Mysterious Island (1865-1869) take place in the same verse, the latter being a sort of sequel to the former two. However, in the latter book, Verne states that the story takes place 20 years after 20.000 Leagues, and 12 years after Captain Grant's Children. He even makes up entirely new dates for his previous novels.
 * The Name of the Wind is usually very good with mathematical consistency, but apparently Rothfuss dropped the ball when he was writing the scene where Kvothe takes his University entrance exam. The mathematics master asks for the length of the third side of a triangle with a sixty-degree angle between sides of 3 feet and 7 feet. Kvothe's answer, "Six feet six inches, dead even", is accepted as correct... except that the answer is actually the square root of 37, which is slightly less than six foot one inch. Even given that he's doing it in his head, he really should have known the answer was closer to 6 feet than 6.5 feet. Since the square root of 37 is irrational, there's no possible way for "six feet six inches" to be exactly correct, unless they use some strange sort of inch that's an irrational multiple of a foot.
 * If you look at the family tree in the last chapter of Centennial, you'll notice that Prudence Wolf (1866-1936) was the mother of Pale Star Zendt (1874-1939).
 * Which is still technically possible, alas, as there have been girls younger than age 10 who have given birth to normal, healthy children.
 * Diane Setterfield's The Thirteenth Tale is an otherwise superb, captivating story. Until, that is, two highly intelligent characters (Setterfield says explicitly that these two are brilliant and gives numerous legit examples) notice a geometric shape in a topiary and argue about whether it is a tetrahedron or a dodecahedron. Setting aside the fact that these are obviously different shapes (tetra = 4, dodeca = 12), the author then mentions that the topiary in question has six sides and the brainiacs agree it's a tetrahedron!
 * Tetrahedrons do have six edges though. And the "sides" of a polyhedron are usually called faces.
 * Mercedes Lackey's The Eagle and the Nightingales. Early on, when Nightingale is hired for a job and informed that payscale varies depending on popularity, she's told that the highest payrate anyone's ever earned is a half-royal, equivalent to five gold pieces, and that this is jaw-droppingly impressive. Later, at the same job, she's given a payraise to five royals.
 * That might be the result of an editing error or typo, surprisingly not uncommon in some editions. It's mentioned that the other musicians get paid more than a half-royal, so it wouldn't be hard to imagine that she's actually getting paid two or three and a half royals.
 * Mercedes Lackey's Heralds of Valdemar series has some rather... questionable timelines. The Tedrel Wars, for example. Jadus was a young man when he fought in it, probably 20-25. In the Arrows Trilogy, he dies of old age. Considering that Healers are readily available, that can't have been before he was 60, making the Tedrel wars at least 35 years ago. Problem? Elspeth was born about 2 years after the Tedrel Wars. She's a small child when Jadus dies. So apparently 33 is considered a "small child" in this series?
 * Another problem comes when you consider Skif. His mentor, Bazie, - who fought in the war - says that it were roughly a decade ago. A few years later, Skif goes to the Collegium. At this point it's probably about 15 years after the wars, by what Bazie said. Problem? He gets there before Talia arrives. When Talia arrives, Elspeth is a "small child". Again, she'd be at least preteen if not teenager by that point. Considering Talia is 13 herself, she shouldn't consider a 12-13 year old small.
 * Lois McMaster Bujold's math is usually spot on, but Mirror Dance seems to have been shifted from two years after the previous book to four years after late in production. This caused attendant chronological confusion throughout the novel.
 * Most of the times and dates discussed in Around the World in Eighty Days add up just fine. But in the first chapter, when Mr. Fogg asks Passepartout what time it is, he says it's twenty-two minutes after eleven. Mr. Fogg says that he is four minutes too slow—it is, in fact, twenty-nine minutes after eleven. Where did the other three minutes go?
 * F. Scott Fitzgerald made several errors with the timeline for The Great Gatsby: basically whenever a character mentions a number of years, it adds up a different way. The mystique and uncertainty behind Gatsby's past only partly accounts for it.
 * This is common in the later works by Harry Turtledove, as noted here.
 * Ian Fleming apparently did not take into account the weight of a bar of a gold, as Goldfinger's plan involves robbing Fort Knox blind (after using an atomic bomb to break open the vault) in just a few minutes. The Film of the Book thankfully lampshaded this problem.
 * Len Deighton deliberately does this in Bomber, a fictionalised account of a British night-bomber raid in WW 2 gone terribly wrong, and the fates that befall the various characters. He sets the action on the night of "June 31st 1943" in order to remove the events of the novel from historical reality, and not only hangs a lampshade over it but practically smashes the lampshade over the reader's head.
 * The "Coruscant Nights" trilogy in the Star Wars Expanded Universe has a lot of this. The author intended for it to take place shortly before the original trilogy, but the people in charge decided at the last minute to change it to shortly after the prequel trilogy without bothering with much editing, making many references to past events almost twenty years off.
 * Also the size of clone army. How many of them are there? Three millions? Over how many planets? Guys, you have any idea of how many people participated in World War II?
 * At the beginning of Soon I Will Be Invincible, Doctor Impossible says that there are 1,686 persons with superpowers on Earth and then gives a breakdown by type which sums to 1,778. It's possible there's intended to be some overlap, however.
 * In Charlie and the Great Glass Elevator, there are 150 people in the Transport Capsule. Twenty-four are eaten by Knids, and the letter at the end says there are 136 left. This should be 126.
 * In Buzz Aldrin and John Barnes' Encounter With Tiber, it's stated that the hexadecimal system counts from 1 through E. Hexadecimal uses the digits 0 through 9 and the letters A through F. One assumes the error was Barnes'; with the computer systems of the 1960s, it's very likely that astronauts would've had to learn hexadecimal.
 * In point of actual fact, though, the human interface device of the Apollo Guidance Computer used octal (0-7) notation (for coded verbal strings) along with digital (0-9) notation (for numerical strings). Indeed, in the 60s and for some time after, octal was vastly more popular than hex, as it didn't require any "non-digits", but eventually the convenience factor of working in hex on machines with 8, 16, 32, and 64-bit words won out and hex is much more common today.
 * The writers also failed to notice that 4097 is not a prime number.
 * In Greg Bear's Eon, the protagonists find a source of an "inverse-square force." A couple of pages later, the force is described as increasing in strength as one got farther from it.
 * A calendar hiccup in Remote Man by Elizabeth Honey, features Thanksgiving approximately three-fifths of the way through the book, but sets the climax on November 30, by which point several weeks have gone by.
 * If you bear in mind that Eragon is sixteen when Murtagh tells his back story, then you'll notice that a year disappears in the midst of the same exposition it was mentioned in. Of course, you also have to know that  since Murtagh mentions that his most recent birthday was when he turned eighteen. In fact, you may be losing two years in there, but it could be due to Retcon.
 * The narrator of Frederik Pohl's Day Million is aware that his listeners are from around "the six or seven hundred thousandth day since Christ." He then identifies Day Million as "ten thousand years from now." Even assuming that his earliest estimate is correct (which would be well before the story's 1966 publication), that makes only forty days to a year. Granted, it could be just the impatient Lemony Narrator's carelessness, but you would think at least that fellow author and praising commentator Robert Silverberg would have something to say about it.
 * There's a glaring example in the Weather Warden book series by Rachel Caine. The lead character, Joanne, has a supernatural adult daughter named Imara, who technically doesn't age. In the fifth book, Firestorm, a character comments to Joanne that she is "plenty old enough" to have an adult daughter. Joanne's character is twenty-eight years old, there's no way she could have an adult daughter. Suppose Imara is twenty years old; that would mean Jo had her at eight years old.
 * Threshold by Caitlin Kieran contains a rare attempt of an author to include mathematics higher than arithmetic, but unfortunately falls headlong into this trope. The book lists a regular heptagon (a seven-sided polygon with all sides and angles the same) as an Alien Geometry based on the fact that a regular heptagon is not constructable. However, in geometry, "not constructable" means "cannot be drawn with only a straight edge and compass." It does not mean "cannot be made with any tool known to man" (you can draw a regular heptagon just fine if you have an accurate protractor and ruler) and most definitely doesn't mean "cannot exist in nature" or "seeing one will cause you to Go Mad from the Revelation."
 * In Le Morte Darthur, Lancelot faces a dozen armored knights while unarmored himself. He kills one of them, takes his armor, then kills "the remaining twelve." Malory takes the trouble to name each one individually, so you'd think he'd have remembered to decrement the count.
 * In The Poisoned Chocolates Case by Anthony Berkeley, a character enumerates twelve conditions the criminal must fulfill, and states: "The mathematical odds against their all being fulfilled in one person are... 479,001,600 to 1. And that, mark you, is if all the chances are even ones." Two errors here. Firstly, his calculation is a little way off—it's actually 212=4,096 (he calculated 12! instead for some reason). Secondly, those are the odds against any particular person fulfilling all the conditions, which is completely irrelevant—after all, assuming he's correct in saying that the crime must have been committed by someone fulfilling all twelve conditions, probability that someone could fulfill all the conditions in itself doesn't matter - it's already a certainty. The question should be, "What are the chances that, given that a particular person fulfills all the conditions, that person is the criminal?"—and the answers to the two questions will not, in general, be the same.
 * This particular fallacy is common with real-world prosecutors too. "The chance that the defendant's DNA would match that found on the crime scene by pure chance is less than one to a million" sounds much more convincing than "The defendant is one of ten or twenty people in the New York metro area who's DNA match".
 * The framing story of the Arabian Nights is stated to take place "during the time of the Sasanid dynasty"—that is, no later than the 7th century A.D. Many of the internal stories that Shahrazad tells take place during the reign of the caliph Harun al-Rashid, more than a century after the collapse of the Sasanid empire.
 * "Lord of the Wolves" by Alexandre Dumas is a horror story about a man who is granted unholy powers. However, the first time he uses them, one hair on his head turns fire red. The second time, it happens to two hairs, the third time, to four hairs, and so on. At one point in the novel, it is stated that more than half of his hairs are red. He proceeds to use his powers a few times more, and by the end of the book, only one single hair on his head remains normal. The problem is, if you add 1+2+4+8... together, the number of red hairs almost exactly doubles with every use of the power. So if it is stated that more than a half of his hairs have been turned "evil", he gets one more use of the power *at most*, and no hairs on his head would remain unchanged. This is especially jarring as it would not impact the plot in any way if the amount of hairs changed into red mid-novel was realistic - not all uses of the character's powers are listed explicitly, all that matters for the purpose of the plot is the single remaining hair by the end.
 * In Louis Sachar's Wayside School series, Wayside School has thirty classrooms, but the total number of students is given as 4500. That would be an average of 150 students per classroom, and since we know that the 30th classroom has 29 students...
 * Most of them are probably in Miss Zarves' class.
 * In Ender's Game, Bonzo Madrid commands Salamander Army for the entire time Ender spends at Battle School, which lasts about three and a half years. This requires either that Bonzo made commander impossibly early (younger than Ender himself does), or that he is long overdue for graduation when he fights Ender in the shower (which Graff explicitly denies is the case just a few pages earlier).
 * Lewis Carroll was one author who was actually very good at maths: he was a maths teacher at Oxford. A lot of logic puzzles were worked into his two most famous books.
 * Although a scientist, Isaac Asimov clearly didn't do the math when it came to Trantor, the planet covered by one city in The Foundation Trilogy (and the inspiration for Coruscant in Star Wars). Trantor's population at its height is given as over 40 billion in a single city covering all 75 million square miles of the planet's land area: assuming 45 billion people, this works out to 600 people per square mile, or roughly equivalent to the United Kingdom instead of a city like Manhatten (which has more than 27,000 people per square mile). However, the city is also explained to go a mile down: the available area increases significantly. If there are only 100 levels underground, actual population density drops to 6 per square mile. The problem with Trantor isn't overcrowding, it's finding someone else to talk to. Maybe he used the long scale?
 * Even with other planets being used to supply Trantor with food (and probably remove the garbage) there have to be some pretty sizable facilities for processing, transporting, and disposing of everything. Most major cities try to locate as many of the power plants and water treatment centers outside city limits, but they still take up room somewhere. Even if every inch of the planet is covered in buildings, its impossible for anywhere near 100% of them to be residential structures. I admit that this is reaching a bit, but it might make things slightly more plausible.
 * The prequel books indicates that relatively large areas of the planet are taken up other things than residential structures - power plants, yeast-food production, infrastructure, etc. The Psychohistorians itself gives three examples of relatively large, sprawling non-residential complexes: the University, the Palace and the spaceports (those giant food-transports have to land somewhere, after all!). Still, there doesn't seem to be enough to bring Trantor up to city-level population density...
 * Quentin Compson's age in Absalom, Absalom! From The Sound and the Fury, we know that he commits suicide in 1910. So when Rosa Coldfield first tells him the story, shortly before he goes to Harvard, the year must be 1909. The narration says that Quentin is 20 at the time. But in the appendix, there's a timeline with the birth and death dates of major characters, which says that Quentin was born in 1891 (and corroborates the date of 1910 for his death). For his roommate, we get the birth year of 1890. "Shreve was nineteen," the narration says (which is possible), "a little younger than Quentin" (which is not).
 * It makes sense when you consider that the entire book is about how narrative refuses to tell the truth and how we can never trust history.
 * The 14th century English monks of Crowland Abbey successfully fended off another abbey's attempt to acquire some of their landholdings in court, presenting as evidence a large book alleged to document their historical claims to the property. Apparently the Crowland monks had succumbed to temptation, and to this trope, because the book has since been recognized as a fraud, not least because some of the senior monks mentioned in its history were alleged to have served at the monastery for as long as 148 years.
 * Faye Kellerman's Peter Decker series has been playing a bit loose with Decker's age in order to keep him out of retirement. In the first book, which was published in 1986 and presumably takes place no later than that year, Decker was described as 38-years-old and a Vietnam vet. Rina's two sons are seven and eight. By the 2010 book Hangman, the sons are still in college and Decker is just turning 60. You do the math.
 * David Weber is normally very conscientious about averting this, but a failure to account for the Square-Cube Law resulted in some ships in the Honorverse of the stated mass and dimensions being about as dense as smoke. The stated dimensions were cut by about 2/3rds in later editions to correct for this.
 * Most of the dates in the journal entries of Bram Stoker's Scrapbook Story Dracula make no sense when compared with characters' descriptions of events in the text.
 * In Helm, there are two notable inconsistencies with ages:
 * Marilyn de Noram is first described, through Leland's eyes, as a young woman who "couldn't have been much older than Leland" (two years older, it is later revealed). Later, Dulan de Laal says that Dillan de Laal is "fifteen years older" than Marilyn. In the family tree at the beginning of the book, Leland is 17 and Dillan is 27.
 * It is said that when Dulan was 25, Dillan was 2 and Dexter "a slight swelling in his mother's figure". Dillan is in fact listed as three years older than Dexter (27 and 24), but Dulan is listed as 52—two years older than this would imply.
 * The Bible makes the case that π=3. Twice. Both 2 Chronicles 4:2 and 1 Kings 7:23 describe the building of a large water tank that is 10 cubits (30 feet) in diameter, with a circumference of 30 cubits. An accurate approximation would put the circumference at about 31-1/2 cubits.
 * However, if we take into account that the bowl was said to be 1 handbreadth thick (appx. 1/4 cubit), and then assume the diameter given is the outer diameter (10 cubits), and the circumference given is the inner circumference (30 cubits) we can calculate the circumference over the diameter to be about 3.16, which is close enough considering all units given were whole numbers and the measurement tools of the period were ropes with knots in them.
 * The Gospels of Luke and Matthew both list the genealogy of Jesus, leading back to King David. However, they don't match, featuring several discrepancies. A number of explanations for this have been suggested, but this trope comes in where one lineage is 41 entries long and the other is 77 entries long, which, while not impossible, would be very unlikely even if we assume one lineage is Marys and one is Josephs, or if we refer to Josephs lineage by starting with his biological father or hypothesized 'legal' father in the different versions.
 * The story of Noahs flood lays out the exact size of the ark, at 30x50x300 cubits. While pretty big for a boat (approximately the size of the Titanic, going by a cubit being roughly equal to a yard), this boat had to fit at least two of every animal and seven of some other animals, as well as all of the food necessary to keeps its passengers alive for the whole duration. Possibly justified if God worked some miracle to make the Ark bigger on the inside.
 * O. Henry's The Gift of the Magi begins 'One dollar and eighty-seven cents. That was all. And sixty cents of it was in pennies.' That leaves $1.27 containing no pennies.
 * Deliberately invoked in the Doctor Who Virgin New Adventures novel Conundrum, which is set in a fictional pocket dimension. The Doctor is playing Scrabble and his opponent makes an excellent move which, as the Doctor points out, cannot be made on a standard Scrabble board. And then glares at the 'writer' for making a mistake.

Live-Action TV
"The Middleman: Never ask Sensei Ping about his age, the wrestler mask... Wendy Watson: Or the clan of the pointed stick, I know."
 * Buffy the Vampire Slayer: Joss Whedon has stated that he sucks at math. Among other things, Spike has miraculously managed to become younger with each passing season:
 * In season two, he is stated by Giles to be "barely 200."
 * In season four, Spike tells Willow that he's "only 126."
 * Finally, in season five, the flashback episode "Fool for Love" established Spike as having been changed into a vampire by Drusilla in 1880. As the episode was made in 2000, this would make him 120 in vampire years. We don't know precisely how old he was when he was vamped, but it's more than six and less than eighty.
 * In Season 1 and 2, Angel is mentioned as being 240 and 241, respectively. Those seasons took place in 1997 and 1998, meaning he was born in 1757 (or 1756). However, the flashback in Becoming: Part II shows him being sired by Darla in 1753.
 * Buffy herself has at least three different birth dates, although this is more about continuity than math. In a season 1 episode, we see her school details on a computer screen, with two different birthdates, in 1979 and 1980 (the screen display changes between cuts). Then her gravestones (in the season 1 Nightmares episode, and at the end of season 5) show her birth date as 1981. Besides this, she also said in a season 4 episode that her astrological sign was Capricorn on the cusp of Aquarius, which puts her birthday on or soon before January 19 (which does not fit either of the season 1 birthdays).
 * Fridge Brilliance: When we see her two different birth dates, they are on a computer screen in an episode where a demon has possessed the internet, and has been randomizing various computer records.
 * It happens with the ages of the Friends. For example, Ross' birth date doesn't match with his purported age and his birth date changes throughout the series. The same problem appears when the matter of who is older ensues.
 * Rachel's Longest Pregnancy Ever also counts.
 * Possibly the funniest is that Monica and Rachel were stated to be twenty-six in the pilot, but they didn't turn thirty until season seven.
 * Heroes: Adam was around thirty when we saw him in 1671, which would make his birth year around 1640. But in a season 1 episode, "Takezo Kensei's" date of birth is given as 1584. Even accounting for the fact that Adam was a bald-faced liar, no one in their right mind would have mistaken him for an 87-year-old man.
 * An immortal 87-year-old that doesn't age.
 * Thirty Rock: Liz somehow aged from 35 to 37 between two birthdays. Tracy seems to be an example of this; he says he's 34, but has been married for 17 years and was in a music video (as an adult) in what is presumably The Eighties. However, he later claims that he's 39, and it turns out that he lies about his age, since he has no birth certificate and doesn't know his birthdate. And because he's black, so it's hard to visually estimate. (Note: This is not racist; it was a gag in the series. Tracy has an "illegitimate son" who is actually a con artist who's almost the same age as him, but Liz and Pete can't work this out due to how bad they are at judging African-Americans' ages.) And because he's a Cloudcuckoolander.
 * Stargate SG-1: Catherine Langford says she was twenty-one in 1945. This would make her four in 1928, yet Stargate shows her to be much older (probably about twelve) in that year. Of course, this is hardly the only inconsistency between the film and the series.
 * In Star Trek: Voyager, the Voyager should have run out of torpedoes way before the end of the show based on the few times there was an inventory count mentioned in dialogue, but they somehow were able to keep firing them. They never seemed to run out of shuttles either. Given the speed with which they whipped up the Delta Flyer, it seems reasonable to assume they could replicate parts and assemble new ones, but at a limited rate.
 * In fact, in one late season, Chakotay states that they have a full complement of shuttles despite at least three getting destroyed on-camera. Therefore, it is canon that B'Elanna was rebuilding shuttles, and probably torpedoes as well.
 * The problem there is not that they were building new torpedoes; the problem is that the fifth episode of the show makes it canon that Voyager can't rebuild torpedoes: she has 38 rounds available, and "no way to replace them after they're gone." The showrunners not only retconned this, They Just Didn't Care enough to even acknowledge they'd done it.
 * Even worse, and crossing into You Fail Biology Forever, is the Ocompan reproduction system. Each female can become pregnant and give birth once in her life, and every birth we know of was single, no twins triplets etc... Even if every single female succesfully concieved and elivered a baby at that time, they'd still have a radicly decling population. Which coupled with an Ocompan generation being about 3–4 years. means with those numbers the entire Ocompan race should become extinct in about the life span of an average 24th century human.
 * In the Star Trek TNG Writer's Technical Guide it is stated that starships have industrial replicators for standard parts from which things like torpedoes and shuttles can be assembled. All that is required is energy and bulk matter. The source also states that dilithium crystals that control the anti-matter/matter reaction can re-recrystallized, presumably using the fusion reactors of the impulse drive as an energy source, so all a starship really needs to sustain itself is a source of hydrogen matter, which is abundant in space, and the function of the glowing bits in the front of the engine nacelles is established as Bussard collectors (collecting hydrogen) already in the original series.
 * In the original series episode "Court Martial", Kirk sets the computer to increase sound by a degree of "one to the fourth power." He, of course, presumably meant to say "one times ten to the fourth power". One to the fourth power equals... one.
 * The Occupation of Bajor in Star Trek: Deep Space Nine lasted 50 years and killed 15 million of the 3.8 billion population. This ~300,000 deaths a year is portrayed as a horrible act that involved genocide, yet is actually a lower death total than modern countries with tiny fractions of the population have during peace time. Medical science is amazing in the 2300s, but it's still a ridiculously count for a "genocide".
 * In the first series of the new Doctor Who, Rose travels back to November 1987, where we see her as a baby of about six months old (according to series 2's "Rise of the Cybermen") -- except there are repeated references to her being 19 in her first season, which starts in March 2005.
 * Also, the Doctor has suddenly gotten younger in the new series. Seven said he's 953, Eleven is forty-six years younger than that in "Flesh and Stone", and Nine said he's been traveling in the TARDIS for 900 years and also claimed that's his age. On the other hand, Romana flat-out stated that he was lying about his age and appeared to have facts to back it up.
 * And remember when the Doctor was stranded on Earth for a whole century? Apparently he doesn't.
 * Steven Moffat, the man in charge from 2010, has openly admitted that "in his mind," the Doctor has lost count and says 900 because it sounds coolest. Also from Moffat, "this is a man who logically should have no idea how old he is." There's some justification for this, since the Doctor had his memory almost totally erased at least four different times and we can't be sure he recovered it all each time.
 * Ron Moore's standard explanation for chronological inconsistencies in Battlestar Galactica is that he's bad at math.
 * 24: The first season takes place "two years to the day" after Jack Bauer led a covert strike in Yugoslavia during the 1999 Kosovo War. It is also a presidential election year.
 * Degrassi, in all its incarnations, is ridiculous when it comes to this:
 * In the 7th episode of the first season of Degrassi Junior High, Wheels is said to be 14. He has a birthday in the 12th episode of that season. In the first episode of the 3rd season, he is said to be "almost 15", and stays that way until a month after the 11th episode of that season (which was most of a year after the first).
 * Spike has baby Emma at the end of the second season of Degrassi Junior High, which is the end of year eight for Spike. So, when Spike graduates, Emma should be approximately four. At Spike's ten-year reunion in the first episode of Degrassi the Next Generation, Emma is only twelve.
 * In the first episode of Degrassi the Next Generation, the class reuniting is said to be the class of '91—even though the classes reuniting (yes, there are two classes reuniting for some reason) should be the classes of '92 and '93, respectively. The writers seemed to have based this off when Degrassi High ended, not taking into account that it ended with the characters in years ten and eleven.
 * After death, he is said to have been born in 1990, making him sixteen at the oldest. He was in year twelve, in which you are normally seventeen or eighteen.
 * Certain aspects of The Middleman's timing are... odd, though probably not outright contradictory. Consider the following:
 * The show seems to be set in 2009. Middleman '69 says he's been in cryo for forty years. Also, Wendy and Lacey have known each other for five years and met as freshman, and Wendy watched Voyager 2 pass Neptune (in 1989) when she was three, which would likely put her in the college class of 2008.
 * The Middleman was on his high-school football team in 1991. Presumably he was at least sixteen then, so he's at least 34 now.
 * Sensei Ping first defeated a hundred men when he was twice the Middleman's age. So he's at least 68 (and defeated those hundred men when he was at least 68; given how casual he is about the idea now, there's a decent chance he's a lot older). This is kind of weird on its own, especially since he doesn't look anywhere near that old, but maybe that's why he doesn't want people asking about his age. However, he's also been training in martial arts for "two of [Wendy's] lifetimes", and she's 23. He must therefore have been an adult before he started training.
 * We get all this info on his age, all in one episode, and none of it makes sense! Hei, wait a minute...

"Will: [talking about the day Kennedy was assassinated] I was playing touch football at recess. Scotty: Recess? I thought you were, like, forty-five when that happened. Lilly: No, you're thinking of when Lincoln was shot. Will: Keep it up. See what happens."
 * The OC: In season one, Marissa's little sister Kaitlin is referred to by Seth as a fifth-grader, yet when the bus returns in season three, she's already in eighth grade, and looks even older, though strangely enough for a series heavy with Dawson Casting, she wasn't—the actress turned fourteen shortly after her first episode aired.
 * The characters spent three years in high school, but Marissa's tenth-grade is mentioned in the past tense in the first season.
 * A particularly bad one happened on All My Children, with the introduction of Kendall Hart, Erica Kane's given-up-for-adoption daughter. The storyline established Kendall as being seventeen, with Erica having given birth to her as a young teenager (she was raped by one of her father's Hollywood buddies). The problem here was this would've made Erica 34 at the most. Erica had been older than that for at least a decade. Once the writers realized their blunder, they quietly Ret Coned Kendall's age to a suitably-vague mid-twenties—which was still pushing things, but acceptably so. It also led to the classic example of Inverse Dawson Casting: The change meant that sixteen-year-old Sarah Michelle Gellar was playing a twenty-something.
 * In one episode of Criminal Minds, Aaron Hotchner recollects meeting his wife when she was a tenth grader and he a junior. The impetus for his memory is a yearbook: "Reflections 1987." If Hotch were a seventeen-year-old junior, that would make him born in 1970, and thus approximately 35 at the time of the episode. Previously, viewers were told that Hotch had been a prosecutor before joining the FBI. So, if he graduated high school at eighteen, then college at 22, then law school at 25, and immediately became a prosecutor, then maybe he could join the Bureau at 27. That gives him eight years to not only be promoted to the BAU, but to become the Unit Chief. Considering how hard the BAU is to get into, a fact which the show points out, that seems... unlikely.
 * A year later, Agent Rossi is introduced. We're told that he last worked for the BAU ten years before, and that Hotch was on the team with him. Meaning that Hotch would have had to join the BAU at age 27, and thus the FBI itself decidedly earlier, meaning he would have to... oh, we give up.
 * In a later episode, Hotch's medical chart lists him as being 43, four years older than his yearbook leads us to believe. Or maybe three years older, if it was his wife's yearbook.
 * In Kath and Kim flashbacks show Kath being pregnant with Kim in the seventies, while Kim, Sharon, and Brett are teenagers in the eighties. Considering the show (aired from 2002) was set in the present day, Kim ought to have been born around 1978 and met Brett in the mid nineties. Rather than actually failing at math, the writers probably just thought the eighties were a better source of jokes (regarding fashion and whatnot) than the nineties.
 * You think a show that relies so heavily on Flashbacks and Time Shifted Actors as Cold Case would be free of this, right? Wrong! The conflicting data about the age of the most senior detective, Will Jeffries, was eventually Lampshaded in sixth season episode "November 22nd":


 * In Veronica Mars, Veronica is hired by a boy, probably a freshman, to track down his dad, who was supposed to have run away. She reads his permanent file and learns that the kid's dad has been dead since he was in the first grade. It is also mentioned he's been gone for seven years. Later, they discover that he really is alive, and when they find him, the son says, "I've thought you were dead since I was eleven!" So either he was a really late first-grader, or he's a really old freshman (the kid looks, at the most, fourteen or fifteen).
 * He didn't mean that his dad went missing when he was eleven, he meant that eleven was the age that he started thinking he was dead.
 * In one episode of Bones, it's revealed that Brennan had a relationship with her advisor in Chicago when she was 24. In another episode, she's said to have been born in 1976, and been working at the Jeffersonian Institute since 1998 (when she would have been 21 or 22). In another episode, she talks about identifying bodies after the Branch Davidian siege. Which means that she was at least 23 by 1993.
 * In another episode, Angela kept asking her co-workers to donate money to save a cute piglet from becoming bacon. She claimed doing this would cost $1500, yet if buying and rearing pigs cost that much, pork would cost a lot more than it does.
 * A whole pig that's already dead costs about half that.
 * Johnny Drama's age in Entourage rises and falls between episodes. In some, he was in high school with Vincent, his younger brother, putting him at about three years older; in others he could drive and legally buy liquor while his brother was still in grade school.
 * Drama is not the sharpest tool in the shed. It's not inconceivable he failed a grade several times making him 21 and still in high school. Perhaps for some odd reason Drama and Vince went to Highschool in Canada for a year in which case he only had to be 19 in most of Canada or 18 in Alberta.
 * NCIS is pretty bad for this. Gibbs' backstory features a hilariously implausible timeline and Tony's age changes every other time it's mentioned.
 * Oh, NCIS: Los Angeles. G. Callen was in foster care from a very young age until, presumably, he was eighteen. During that time he was in 37 foster homes. All right. However, the longest he ever stayed in any home was three months. Even if he stayed in the other 36 homes for two months and 29 days each, that still doesn't come anywhere close to a minimum of thirteen years. And it's specifically mentioned that he usually only spent a few weeks to a matter of days in each place. Possibly there was some time spent in group homes, or something to that effect, but still. No.
 * Eureka has time travel between 2010 and 1947. The time travel is explicitly said to be possible only when an eleven-year solar flare cycle is at its peak. 63 years are between the two points, too many for five cycles but not enough for six.
 * Charmed has a demon who appears every 1300 years on Friday the 13th (who conveniently shows up in the thirteenth episode of the series, looking to kill thirteen unmarried witches). But in the seventh century AD, when he would have last appeared, our currently-used Gregorian calendar wasn't even a twinkle in Pope Gregory XIII's eye; in fact he wouldn't even be born for another 900 years or so. Was the demon's last appearance on the day that people at the time would have called Friday the 10th, or does he adjust his date of appearance to conform to the currently extant calendar (in which case he could theoretically be beaten by changing to a 31-month calendar in which every month has only twelve days at most)?
 * Boy Meets World begins in the fall of 1993. Cory is 11 and in 6th grade, his older brother Eric is 15 and in 10th grade, and their little sister Morgan is 5 and in kindergarten. Next year/season, Cory is 12 and in 7th grade, Eric is 16 and in 11th grade, and Morgan is 6 and in 1st grade. So far, so good, right? Not quite. Towards the end of the second season, Cory is suddenly 13 and presumably in 8th grade, considering final exams were taken in a second season episode way before the finale. When the third season begins, Cory and his friends are 14 and in 9th grade, although Eric's aging process is still consistent (17 and in 12th grade). When Morgan suddenly re-appears, she's 8 and in 3rd grade. At the end of the third season, Cory, Shawn, and Topanga end the school year as 15-year-old 10th graders. Eric's age and grade, however, are still consistent (he graduates high school at 18, as he was supposed to). The fourth season begins with Cory, Shawn, and Topanga as 16-year-old 11th graders. Eric is an 18-year-old college reject who's trying to find his place in the world. Morgan's age and grade are ambiguous. The fifth season begins, and the aging process is fluid once again. Cory, Shawn, and Topanga are 17-year-olds in 12th grade, and Eric is a 19-year-old college freshman. Next year/season, Cory, Shawn, Topanga, and (new cast member) Angela are 18-year-old college freshmen, and Eric, Jack, and (new character) Rachel are college sophomores. The year/season after that, however, Eric, Rachel, and Jack are college seniors. I guess the writers decided to act as if Eric's year off between high school and college never existed, and he began his freshman year in September 1996 instead of September 1997. Also, at the beginning of the seventh season, Morgan is suddenly thirteen, dating boys, and always on her period (as shown by her attitude).
 * Glee has an episode ("Acafellas") in the first season where Kurt drives himself, Mercedes, Tina, and Rachel to see Vocal Adrenaline perform. In Ohio, teenagers can get their permit at fifteen and their license at sixteen, and can't drive more than one other person until they're eighteen, which is pretty consistent with most states. Of course, it's possible that Kurt's breaking the law, but given his idea of chaos
 * Probably a case of SoCalization
 * Another episode ("Night of Neglect") has Will Schuester telling the kids they need to sell taffy to raise money. He reasons that, in order to raise $5,000, if each taffy is 25 cents, they need to sell 20,000. He writes it as "5000 x .25 = 20,000." However, one of the producers did later say that the math was intentionally terrible because they thought it would be funny.
 * Finn's father apparently died in Operation Desert Storm, when Finn was a baby. However Finn was a sophomore in the 2009-2010 school year meaning Finn was born at the earliest 1993, Desert Storm ended in 1991, so unless we find out that Finn's mother lied about it it's either this or You Fail History Forever.
 * She did lie, as she later reveals Finn's father came back from Desert Storm with PTSD, a drug problem, and an abusive personality. This would be when Finn was conceived.
 * Power Rangers has screwed up a couple of times. Tommy was seen graduating high school as part of the Class of 1997 at the start of Power Rangers Turbo. By Power Rangers Dino Thunder, he's got a PhD, and is a teacher at another high school - where his students have a prom at the end of the season - for the Class of 2004. It's impossible to get that qualification, have time to do the research which led to the creation of the bad guys, then become a teacher, in only seven years.
 * Then in Power Rangers Samurai, Bulk is shown bringing up teenager Spike, the son of his best friend Skull. He's about 14, which would mean he was born during the Zeo or Turbo seasons. Wasn't mentioned at the time!
 * In the first episode, of Young Blades, Louis XIV is stated to be 15 years old. However, a later episode gives the date as 1652. Since Louis XIV was born on September 5, 1638, he would be either 13 or 14 in 1652.
 * In the first episode of Stan Lee's Superhumans the narrator mis-applies the physics formula F=MA to a strongman's strength by implying that the body weight of the Weightlifter would limit the force he could produce. This is a totally false premise that has nothing whatever to do with the formula, which is actually used to determine the force required to move an object of mass M with an acceleration of A. Since the context was the weightlifter bending objects like frying pans and wrenches, A is zero. The object being bent is moving very little, if at all, but is simply being deformed.

Music
"We gotta flee to Panama, but wait it's half-and-half\"
 * The song/story "A Billion Baseballs" by the Green Chili Jam Band does multiple calculations related to these baseballs and gets almost all of them wrong. For example, it says that this many baseballs placed on the ground would take up a giant square "eighty miles around." Since baseballs are three-inch diameter objects, a square of 31,623 by 31,623 baseballs would have a perimeter of less than six miles.
 * Foxy Brown's verse on The Firm's "Affirmative Action" contains this horrible bit of addition:

Keys is one and two-fifth, so how we flip Thirty-two grams raw, chop it in half, get sixteen, double it times three We got forty-eight, which mean a whole lot of cream Divide the profit by four, subtract it by eight We back to sixteen, now add the other two that 'Mega bringin' through So let's see, if we flip this other key Then that's more for me, mad coke and mad leak Plus a five hundred, cut in half is two-fifty Now triple that times three, we got three-quarters of another ki
 * The Shooter Jennings concept album "Black Ribbons" is set in a not-too-distant future dystopia. The narrator, the Will 'o the Wisp, makes reference to a real book published in the 1970s, stating it was written forty years ago, putting the date around 2010, when the album was released. This could be a case of a changed past, but then he mentions Obama's election and says everything went to seed after that. Either Obama's term was cut short, or the math is wonky. This is made worse by the fact that Stephen King wrote the narration.
 * Or the narrator is unable to count it right because everything went to seed after that - including the math curriculum.
 * The Rage Against the Machine track "Down Rodeo" includes the classic hook: So now I'm rollin down Rodeo with a shotgun / These people ain't seen a brown-skinned man / since their grandparents bought one. Even if the song is describing a 1960's Black Panther, which the lyrics seem to suggest, that's over a century since the end of slavery, making it extremely unlikely that anyone there would have been old enough to see one purchased.

Newspaper Comics

 * The Dick Tracy story "The Man of a Million Faces" features a string of bank robberies committed by celebrity lookalikes—or is it a single master of disguise? The story makes a big deal of using known heights of objects to measure (they mean "calculate") the perpetrator's height, which turns out to be a consistent 5'11". Yet a previous diagram shows a line labeled 69.9" going over his head with room to spare.
 * Another Dick Tracy story involves thieves stealing small-valued coins from parking meters. Now, $2 in nickels is only 40 coins, far less than the large handful shown, whereas when Larry throws $20 in pennies onto his mother's stomach, the size of the bag pictured is about right, but that would seriously hurt.

Tabletop Games

 * In the Warhammer Fantasy Battle setting, different factions use different calendars, which contain references to the standard Imperial Calendar to allow for calibration by sufficiently interested fans. Unfortunately, when the same events are compared in different calendars, the dates are all out by one year, as the writers forgot that the first year of a reign starting in, say, 2000, is 2000, not 2001.
 * In one edition of Warhammer Fantasy Roleplay, it attempts to make the point that magic users are rare by claiming 1 in 1,000 people have some magic ability, 1 in 1,000 of those are true wizards, and out of those, only 1 in 10,000 are cut out to be battle mages. That means a planet with the population of modern Earth would probably have one at most. It seems unlikely a medieval setting would have even that many.
 * Warhammer 40,000 had the authors all too often try the cheap path of impressing the reader with amount of zeroes, only to lose the track of zeroes themselves. There was contradictory data on various statistics of Imperial spaceships alone, and that's before you look at orbital bombardment. The usual way to handwave the mess is appeal to Unreliable Narrator, but this is applicable only to the part that are, well, narrated - a novel may get a pass, but a sourcebook doesn't have much of that.
 * In the current Codex for the Black Templars space marine chapter, the introduction states that the whole chapter is divided into "crusades", of which there are "usually no more than a few", comprising fifty to several hundred marines. This would be fine if the Templars were a chapter of one thousand, following the Codex Astartes, but the same page—nay, the same paragraph—says that there are between five and six thousand Templar marines. The numbers just can't work with only a "few" crusades, unless a "few" is pretty large or each is over a thousand strong.
 * It does work out if you interpret "crusades" as meaning "in active combat". There could be any number of Templar companies doing routine garrison duties or recruitment.
 * After the Horus Heresy, the ten remaining Space Marine Legions were split into a thousand chapters, so each Legion would (on average) be split into a hundred chapters. In the same book, the Ultramarines Legion is claimed to have by far the most offsplit chapters, at twenty-three.
 * Post Horus Heresy there were actually nine remaining legions, and the thousand chapters is both an approximation, and valid for "current" time in the 40k universe (M41), not immediately post Horus Heresy (M31). There have been loads and loads of foundings of new chapters in the intervening 10,000 years.
 * For the "Ace In The Hole" miniadventure included with the Star Ace GM screen, the PCs are supposed to rob a casino blind as revenge for the casino owner putting a bounty on one of your NPC buddies. You're also supposed to leave 5000 chips (game currency equivalent to $5 million US) behind to cover the gambling debt that started this whole mess. The scenario uses random rolls to determine just how much money is in the casino at any given point, but if you assume maximum rolls there's only 700 chips in the whole joint. You can't even make up the other 4300 chips by robbing the 1d10x100 customers, because all gambling is done with house scrip instead of coins. Not to mention that the casino owners have put a 1000 chip bounty out on that NPC's head. It's fairly simple for the PCs to bluff their into the casino office by producing fake evidence that they'd just earned the bounty... which the casino owner isn't able to pay because he doesn't have that sort of cash on hand. The Star Ace game assumes that all currency is "hard" currency; computer credits and the like aren't used.
 * Using the completely bonkers equations given in Role Master's first companion to determine one's character's height, weight, bust size, waist size and shoe size (you know, in case you're a very, very anal GM and your dungeon has Boots of Speed in size seven -- this is Role Master, after all, the Dwarf Fortress of pen and paper RPGs) you would always end up with a monster who had feet twice as long as his waist or somesuch.
 * Likewise, the original 1st Edition AD&D tables for character height and weight failed to link these two characteristics, meaning the shorter your PC was, the fatter they were, and tall characters were built like string beans, if not dental floss. Furthermore, the weight tables for demihumans were way, way too extreme, meaning your dwarf hero could've had the approximate density of osmium.
 * Dungeons & Dragons
 * 3rd edition authors decided to "balance" things for dungeon crawl. A ten-foot ladder costs five copper, like in the previous editions. A ten-foot pole costs two silver (twenty copper). Assuming you sell for half the market value, there's good money to make in buying ladders and taking them apart. Also, ladder makers are apparently poor, poor people.
 * And an iron ingot is worth more that an equal weight of iron buckets.
 * Similarly, in the game In The Labyrinth, a full wineskin costs $2, while an empty one costs $3. Bottoms up!
 * The numbers for the fireball spell were a little off. The shape was controlled by the caster and the volume was fixed, and the resulting blast tended to be the size of Pennsylvania.
 * Some of the early Star Wars tabletop RPG material, namely the Imperial Sourcebook, refers in much detail to the Order of Battle for starships. One of these configurations is called a Fleet Bombard, which would include two System Bombards, which in turn include three Bombard Squadrons, which in turn would include two Torpedo Lines, which each usually consisting of two Torpedo Spheres. That means a single Fleet Bombard should have about 24 Torpedo Spheres. Later in the book, however, they talk about how there are only six Torpedo Spheres in operation in the entire fleet.
 * Considering the giant pile of bile that is FATAL, it should be no surprise that randomly generated body parts can have negative proportions.
 * Though the problem that best fits here is success rolls. First you roll percentile dice to find the number to roll against then roll percentile dice to see if you get a higher number. The probability that roll #1 is greater than roll #2 is, of course, always 0.495 (1/2 of 99/100 that rolls are not equal).
 * In Werewolf: The Apocalypse, when a Garou and their kinfolk partner conceive, the likelihood that they will produce a Garou child is only 10%, compared to a 90% likelihood that they will produce a kinfolk child. That means that each Garou-kinfolk couple would have to have at least ten children to keep the Garou population stable!

Theatre

 * In Gilbert and Sullivan's The Sorcerer, Sir Marmaduke brags that he can trace his family back 7000 generations, all the way to Helen of Troy. Considering that Helen of Troy was around 3000 years ago ...
 * This is probably more a mistake of the character than the writers. They loved mocking the grandiose claims of British nobility. In The Mikado, Pooh-ba claims he can trace his ancestors back to the very first "protoplasmal primordial atomic globule."
 * The Phantom of the Opera takes place in 1881. Its sequel, Love Never Dies, takes place ten years later, in 1907.
 * Phantom itself has had its share of chronology issues—the dates of the prologue, the principal action, and the death of Christine's father have shifted several times since the show's inception, and trying to reconcile all three dates with each other—not to mention the ages of the characters or the history of Paris and the Palais Garnier—has resulted in headaches for many a phan. A brief discussion on the subject can be found here.
 * The Pajama Game has a song named "Seven and a Half Cents" involving the singers detailing what they could buy with that raise over a given number of years. For the last figure for ten years, they forget to carry when multiplying, resulting in the wrong answer.
 * The stage version of Hairspray suffers terribly from this. It starts on a Monday in "early June", 1961. It ends on June 6, 1961. However, around 10 days pass between the beginning and ending scenes, and this problem is compounded by the fact that the first Monday in June, 1961, was the 4th. Admittedly, this problem is not particularly noticable unless you have a copy of the libretto for the show and too much time on your hands.

Video Games

 * Ace Attorney simply tends to ignore precision when it comes to ages and years, despite taking the time to add dates everywhere. Months will pass between cases of a game, yet no one ages... except between games, where everyone gets another year. It's resulted in at least one age contradiction, with two cases Edgeworth appears in. However, the fourth game is the worst; the game mentions constantly that it occurs seven years after the end of the third game. But the dates are all set for 2025, seven years after the beginning of the third game, which started in late 2018 but spilled into 2019.
 * Possibly justified since it's implied Phoenix himself writes down everything in the court record. That also explains how the only age change that is plausible is his own.
 * The Halo universe has never kept the number of Spartans straight. Thirty-three survived the "augmentation" process intact. By the time of the battle of Reach, at least thirteen had been killed, gone missing, or retired, for a total of thirty still active.
 * In Deus Ex one (supposedly) timed event give you 24 hours to live. A helicopter takes you from one location (Liberty Island) to another (Hong Kong) in a trip that is based on context to be less than eleven hours (23 hours left before the flight, twelve hours have passed a decent amount of time after it). Google Maps gives the distance between the two at 8,047 miles, and the max speed ever recorded on a helicopter was 249.1 mph. To make the trip the helicopter in question would need to go over 730 MPH (almost three times the record) without stopping.
 * This may be a subversion, as the helicopter involved is heavily implied to be advanced tech even for the setting.  That said, you would think even a dying protagonist would question why he hasn't died or seen sunrise before landing.
 * There are so many Writers Cannot Do Maths moments with dates in Deus Ex that it's difficult to tell when it's supposed to be. Guesses have ranged between 2045-2065. 2052 does seem to be the most consistent, however, and is the one used by the sequel.
 * In Jak X, it's confirmed that Jak and Keira are only one year apart in age. So,  A common fan theory was that Keira was adopted, but that was debunked once Samos mentioned that she was "just like her mother." The new theory is that Keira's mother was single and married Samos. If she is indeed Samos' biological daughter, as the series seems to treat her, then it falls under this trope.
 * In The Sims 2 & 3, there are a lot of these. For example:
 * The Sims 2 & 3 made Michael Bachelor into Bella Goth's older brother, but in the first game she already had a child, while he was just out of college. And the Sims 3 was supposed to be about 25 years before the first game, yet Michael is a 20-year-old man.
 * Dina Caliente in the second game has a memory of having her first kiss with Michael Bachelor, despite the fact that she married him when he was an elder, which would mean her first kiss was with a middle aged man.
 * Mortimer and Cassandra's memories say that she was almost, but not yet, a teen when Alexander was born, and therefore only twelve years older than him. But if she's a kid in TS 1 and there's 25 years between the two games, she must be pressing thirty by TS2, and Alexander should at least be in his late teens, not a child as he's presented.
 * Kaylynn Langerak shows up in TS3 as a child about the same age as Mortimer Goth. There's fifty years between TS3 and TS2, and Mortimer is fittingly an elder by that time. So why is Kaylynn barely into adulthood?
 * Brandi Broke starts TS2 pregnant with her third child by her late husband, who has her husbands genetics and is recognized by the game as his. Yet her husband died before her second child was born, meaning he somehow sired the third one after he died. In-game, the third child isn't actually his son. The baby is always a boy because Brandi is pregnant with herself, and a character who is pregnant with themselves always has an opposite-sex child.
 * Sierra's Police Quest has a very confused time line. The game was made in 1987 and is apparently supposed to take place in 1983, but there are some references to 1986, such as Hoffman's gun, which was reported stolen three years after his arrest. Police Quest 2 then makes this worse by taking place one year after the original, but mostly seems to refer to 1987 (although there are a few references to 1984 in there). Then in Police Quest 3, the computer records indicate Sonny Bonds was hired by the LPD in 1985, which is after the date that Police Quest 1 was supposed to take place, and is even weirder considering that in Police Quest 1, it was stated that Sonny was already a fifteen year veteran on the force!
 * In Team Fortress 2 various items have descriptions saying it does an action (attacking, switching weapons, revving up a minigun) some % faster or slower, which means the time it takes to complete that action should be divided by the sum or difference of 1 and a hundredth of that percentage (for example "50% speed increase" would mean dividing the time by 1.5, "20% speed decrease" would mean dividing the time by .8). However, those numbers are instead what percentage of the original time the difference in time is (so "50% speed increase" means multiplying the original time by .5 and "20% speed decrease" means multiplying the original time by 1.2)
 * The Baldur's Gate series is centered around the story of the mortal children of the god Bhaal, who foresaw his own death during the coming Time of Troubles and produced a number of mortal children to ensure his resurrection through them. In the last installment, Throne of Bhaal, you learn that an important Bhaalspawn character, who is known to be about twenty-one at this time, was being held as a baby by a cult of Bhaal worshippers who intended to sacrifice Bhaalspawn children to raise Bhaal or something. This all makes perfect sense until you remember that the Time of Troubles was given as having been about a decade ago. This means that the cult was operating about ten years before Bhaal died, which makes its actions nonsensical or at least unexplained.

Visual Novels

 * Narcissu and its sequel, Narcissu ~ Side 2nd, have trouble keeping Setsumi's age straight. In Side 2nd (which takes place during the summer of 1999), Setsumi is referred to as being an Aquaries (thus born in late January or February), and it is alternatively claimed that she is fifteen years old, or that it is her fifteenth summer (which would make her fourteen). The original game (taking place in late January and early February of 2005), claim that she is 22, but adding the years up from her claimed age in Side 2nd indicate that she should be within a month of either her 20th or 21st birthday, depending on whether one uses the "15th Summer" claim or the "15 years old" claim.
 * Additionally, in the prologue of Side 2nd (which takes place between the two stories), Setsumi makes a remark about how she's been hospitalized on and off for a decade. The main story indicates that she was first hospitalized sometime in April 1997, thus making it impossible for the claim of a decade to be correct.
 * In Higurashi no Naku Koro ni, Hinamizawa has a population of roughly 2000, yet there are only 20 pupils in the village's only school. Realistically, around one tenth of a population would be of school age. (The TIPS say that half the children in the village go to school in Okinomiya instead, which helps a little, but isn't nearly enough.)

Web Comics

 * Averted in Schlock Mercenary, especially where explosion yields are concerned. E.g. here. The author even accounted for the equal amount of anti-matter that would be converted to energy when taking 320 milligrams of anti-matter and converting to the equivalent explosion powered by TNT. (Quick link to the maths.)

Web Original

 * In Coach Z's music video "Rap Song", a featured guest singer proclaims, "My name is Tenerence Love, plus my name is Tenerence Love, divided by my name is Tenerence Love, equals my name is Tenerence Love...remainder 3." Depending on how the operations are grouped, the value of "my name is Tenerence Love" could be -1 or it could not exist at all.
 * If we let "my name is Tenerence Love"=x, we have (x+x)/x=2=x+(3/x), since "remainder 3" means the undivided portion of the fraction. Rearranging, we get the statement x^2-2x+3=0, and by using the quadratic equation we find the value of "my name is Tenerence Love"=1+sqrt(2)i or 1-sqrt(2)i.
 * The Nostalgia Critic, in his End of Days review, as part of a long string of math equations, infamously uttered that "9+9+9=21". That's about as basic of mathematics as one can get and he screwed it up. He later apologized for it in one of his "Fuck ups" specials.

Western Animation

 * CatDog: In the episode "Smarter than the Average Dog" when Cat becomes a genius he exclaims "Circumference equals Pi radius squared!" Pi radius squared is the area of a circle, not the circumference.
 * SpongeBob SquarePants: Spongebob has one driving test a year, according to one episode. Yet another claims he has failed the test over 1,000 times. Either they did not do the math, or he's a sprightly 1000-year-old.
 * However, in another episode, Mrs. Puff really wants Spongebob to pass this test because it is the last one this year and otherwise he will be in her class again, implying multiple tests per year.
 * Let's not forget his 2000-and-something Employee of the Month Awards.
 * In the Movie, he had 374 consecutive Employee of the Month Awards. Assuming he was 16 when he started working at the Krusty Krab and worked all 12 months a year, this would put him at 47 at the least.
 * Let's not forget that he's a sponge.
 * And let's not also forget the numerous times that the narrator says "20 million years later" or some such ridiculous period of time. ALL the characters have got to be at least a 1,000 years old by now.
 * One episode shows a brief shot of Spongebob's driver's license in a dream. If that is anything to go by, he was born on July 14, 1986. In order for all of this to make sense, the show would have to be set in the distant future if his alleged DOB and the above details are taken into account.
 * The Time Travel episode of Darkwing Duck has the eponymous hero go back to the 1950s, when he was in elementary school. A different episode seems to depicts him attending high school in the 1970s, if the clothes and hairstyles of the students are any indication.
 * That show has about three different, contradictory explanations for how DW became a hero.
 * In a Powerpuff Girls episode, the girls traveled back in time to the 1950s and met The Professor, Ms. Kean, and Sara Bellum in elementary school. That'd make those characters in their mid-to-late fifties, and they just don't look it.
 * Mayor is probably in his eighties, as his flashback was set in the '20s. Unless it's normal in their universe, he shouldn't look that lively and hopping at his age.
 * This is why the MST3K Mantra is especially useful for children's cartoons.
 * In American Dragon: Jake Long, Jake's grandfather was single in the mid-1970s, but his mom was in high school in the mid-80s, and Jake was fourteen by the time the show started in 2005.
 * A scrapped version of "Homecoming" was supposed to reveal that Lao Shi had Jake's mother out of wedlock with Chang. You can probably guess why Disney vetoed it.
 * Mostly avoided in Futurama, where several of the writers have PhDs in mathematics. Hell, they'll take a breaks at some points in the DVD commentaries just to talk about math.
 * Notably averted in The Prisoner of Benda where one of the writers ended up proving a new math theorem. See here
 * Constantly made fun of with Bender's composition materials. He often remarks how he's 30 or 40% of something (Iron, Titanium, Scrap Metal, Lucky Horseshoes, Dolemite, Zinc, etc) which accumulatively sums up to between 150% and 330% or more!
 * The citizens of Mainframe in ReBoot exist inside a computer and, as such, live their lives at a much faster rate than we do; i.e. a nano in Mainframe is a minute/hour to a real person, a second is comparable to a day, a minute to a month, etc. However, time runs faster inside the Games, which is how Enzo and AndrAIa could grow up in Season 3. So... how can the User play games so quickly? Shouldn't games change time in the exact opposite manner?
 * Enzo and AndrAIa aged because they were on the Net proper, and cycled more quickly. Due to the effects of frequent games and the consequences of a user win annihilating all of the inhabitants,, it's implied that the User is running an old computer that can barely run the games. That said, some of the games progress in real time, implying that ReBoot math fails on other levels: the User is invariably shown moving quickly relative to the sprites (implying the in-game time is sped up, as the above comment suggests, such that every character in a game should be aging at an accelerated rate in each and every game), and implying that the User's games only take a few minutes to play, start to finish.
 * In Beauty and the Beast, according to a lyric in "Be Our Guest", the castle has been under the Enchantress's curse for ten years. Since the rose stopped blooming once the Beast turned twenty-one, this would mean that he was eleven years old when he was cursed, and this does not match up with the stained glass windows in the prologue or the portrait of his human form in the West Wing.
 * An alternate interpretation of "...would bloom until his 21st year" as the rose blooming for 21 years—that is, until his 21st year of enchantment—also clashes with the "ten years" lyric, since 11 more years obviously didn't pass between the song and the last petal falling at the end.
 * On the other hand, the lyric in "Be Our Guest" doesn't explicitly say that the castle has been cursed for ten years, but at least ten years. "Ten years we've been rusting" could imply that they used to still serve the beast and only gave up hope ten years ago. This however would mean that the prince had been cursed for 21 years in addition to his age before being cursed would lead to him being probably in his late thirties, much older than Belle. A tad weird, but after all this is France.
 * Another issue up for debate is the span of time over which the main events of the movie take place. the movie appears to start in autumn, then quickly goes into winter, the famous title song dance scene takes place on a warm night (complete with crickets chirping), yet that same night after Belle is released, there is still a foot of snow on the ground and even LATER that same night, there is a torrential thunderstorm, sugggesting that it is spring, which means Lefue would have been waiting for Belle and Maurice to come home, AND Maurice was in the woods alone, for the entire winter.
 * Batman Beyond originally took place in the year 2039, being referred to as taking place forty years after the end of Batman: The Animated Series (1999). Later on, the creators of the show announced that the show takes place fifty years after the end of Justice League Unlimited (2006), meaning that would place the date at 2056 instead. However, the characters make explicit references to events that happened in the previous series as being no more than forty years ago. Certain sources even give Terry McGinnis a birthdate of 2023. And now, the 2010 Batman Beyond comic series once again confirms the date as being 2039 and thus only forty years passing.
 * In an episode of Dexter's Laboratory, Dexter tells his computer to decipher a clue Dee Dee left to find something she stole from him, namely "r squared". The computer then figures it meant to check the pies his mother made, because pi(e) equals r2. But that's not true, and it was probably supposed to be how pi times r2 equals the area of a circle.
 * It appears the Total Drama writers have problems with basic counting in some instances: for example, whenever Chris has to count the amount of campers left during a challenge (in the second episode of Island). Or, when they said the tenth camper to arrive on the island was "contestant number nine", which may be the root of the Trent problem from season two.
 * They also can't seem to figure out exactly how much time passes between episodes. Sometimes, the characters say it was a week since the last episode, which would require Island to last 24 weeks, not eight. It's usually said that there are three days between challenges, though, which still means about 72 days, or about ten weeks. For that matter, the time counter in the second episode means that that episode's challenge lasted for three and a half days, and that some of the campers slept continously for most of that.
 * Jane from The Jetsons is thirty-three, and her oldest child is seventeen. Unless adopted, she had her at sixteen years old. We doubt it was intentional, since the series is from the '60s.
 * It could be as 18 and 19 were considered prime marriage age, and 16 is still legal today. In fact, in the sixties, dropping out of high school to get married (especially if hubby is joining the army) was considered quite normal. This is probably more a case of Values Dissonance.
 * Two Friz Freleng cartoons from the Warner Bros. stable have this problem. In Boulevardier From The Bronx (1936), Dizzy Dan's team the Giants is already up 2-0 when they score two additional runs (on an inside-the-park homer and a four-base error) which made the score 4-0. But in the bottom of the ninth, where pitcher Dizzy Dan intentionally walks the bases loaded just so he can strike out hayseed Claude (who winds up hitting a grand slam), the scoreboard reads 3-0. Then in 1945's Baseball Bugs, In four innings the Gas House Gorillas score 96 runs against the Tea Totallers. Enter Bugs Bunny, assuming all nine positions for the Tea Totallers and scores 96 runs himself. However, the score in the bottom of the ninth reads Bugs Bunny 96, Gorillas 95.
 * In the Animaniacs cold opening Newsreel of the Stars, it is established that the Warners were created in 1930, and yet their 65th anniversary is celebrated in the show's 65th episode, which aired in 1994, suggesting that they were actually created in 1929. This is lampshaded by an obsessive, overweight nerd in a short called The Please, Please, Please Get a Life Foundation.
 * In a Pinky and The Brain short called Puppet Rulers, the titular mice are living presumably in the 50's and the Brain frequently states that they will freeze themselves for forty years, though later on a caption reads "Thirty Years Later" and the Brain correctly acknowledges them as being in the 90's. Freezing themselves for forty years, as originally stated, would have awakened them in the twenty-first century.
 * In another Pinky and the Brain short entitled Brain Meets Brawn, Pinky accidentally shrinks the Brain four times with his repeated apologies, then restores him to his normal size by hitting (and thus angering) him only three times, though it's possible that some instances shrink or grow him moreso than others, supported by the fact that Pinky's final apology at the end of the episode shrinks the Brain all the way down to the size he was after Pinky had apologized four times.