Unexpected Successor

A character is way down in the line of succession for some political office, to the point where it's reasonable to assume that they're never going to actually take it. Then everyone ahead of them dies or is otherwise disqualified.

Usually this is either because a) some giant disaster occurs, or b) the character in question is evil and killing everyone ahead of them. It can also result from a Succession Crisis.

In a monarchy, this usually means that some distant cousin of the previous monarch is ascending the throne, frequently leading to a great deal of Fish Out of Water humor. Also note that, due to the generally predictable nature of royal succession, it's possible for someone to start their life rather far down the line of succession and still be expected to become monarch someday. For example, Queen Victoria was born fifth in line to the English throne. However, everyone ahead of her was also at least a generation older than she was and old enough that they were unlikely to have more children. So she grew up expecting to be queen, and thus would not be a Real Life example of this trope.

Also contrast Offered the Crown, where the monarch is selected, and Succession Crisis, where ambiguity about who the rightful successor is causes problems.

In the United States, rules for succession beyond Vice President -> President are not written in the Constitution, but are instead legislated by Congress. Currently, if both the President and Vice President are unable to hold office, the Speaker of the House of Representatives becomes President. If he's dead, the President pro tempore of the Senate (traditionally the senior Senator of the majority party) becomes President. After those two, it goes to Cabinet members - starting with the Secretary of State and then proceeding to Treasury, Defense, and the Attorney General. After that the succession goes to the other Cabinet members based on the date of creation of their office, so as of 2010 the eighth person in the official LOS is the Secretary of the Interior, and the eighteenth and last person is the Secretary of Homeland Security. In fiction, the Speaker of the House of Representatives is almost always not a member of the President's political party. Usually the Speaker resigns as President out of courtesy, but for added turmoil, she won't.

During the State of the Union, the President usually sends a member of his Cabinet far away in case the Capitol gets blown up, as the "designated survivor." Usually this is someone who no one would notice actually showing up, like the Secretary of Agriculture. When the Capitol is blown up, the Secretary of Agriculture must go from teaching farmers how to grow peanuts to running a country. Hilarity Ensues.

See also Twenty-Fifth Amendment. Super-Trope of Spare to the Throne.

Anime

 * In Code Geass R2,
 * And at the very end of the series,
 * Katekyo Hitman Reborn. After retiring to Japan, the first Vongola boss had a family there. In the present day, the current generation of Vongola heirs all die separate bloody deaths. The only descendant of a Vongola boss left is Tsunayoshi Sawada from the Japanese family, getting an ordinary Japanese kid into a Mafia family he didn't know he was related to.
 * Shi Ryuuki in Saiunkoku Monogatari unexpectedly became the heir to the throne at age eleven, during a conflict in which four of his brothers killed each other and Ryuuki moved from the bottom of his family's pecking order to being his father's heir. The only other heir, the prince Seien, is in exile. Ryuuki, who isn't interested in being The Emperor and wants nothing more than for his brother Seien to come back, stubbornly resists ruling and acts stupid in the hopes that someone will bring Seien back to take his place.
 * Among the background events in Highschool of the Dead, Presidents of the United States keep getting infected. Each time someone new ascends to the office, he asks himself, "Should we nuke China before our government totally collapses?" One of them decides he should.
 * Friedrich IV, The Emperor in Legend of Galactic Heroes, was fourth in line to the Imperial Throne, with the three front of him both much better politicians and much more ambitious. Friedrich, believing that he'd never get anywhere near the throne, spent his youth being a wastrel and a careless hedonist until events in the Deadly Decadent Court made all prior claimants unsuitable. Even as Emperor, his lack of preparation or even desire for the throne colours much of ruling decisions.
 * from Detective Conan. Sure,, but it was expected that   or   would get the family riches, or even  ...
 * in Umineko no Naku Koro ni. More exactly, in the third arc. In addition,
 * Kinzo Ushiromiya himself is one of these too.

Comic Books

 * The graphic novel Give Me Liberty by Frank Miller. After the death or incapacitation of everyone higher up in the line of succession, the Secretary of Agriculture, Howard Nissen, assumes the presidency.
 * Margaret Valentine in Y: The Last Man was the US Secretary of Agriculture. When all the men suddenly died, she was suddenly promoted all the way to President because everyone ahead of her in the succession was either male or died in the ensuing chaos.
 * In V for Vendetta, following massive social upheaval, the monarch of Great Britain is "Queen Zara." Zara Phillips is currently thirteenth in line for the Crown (although when the comic was written, she was only sixth).

Fan Fiction

 * Marissa Picard and her best friend Clara Stutters discovered that they are princesses of the planet Essex.

Film
"Roy O'Bannon: Aren't you, like, the twentieth to the throne? Lord Rathbone: (annoyed) Tenth."
 * The main character in Kind Hearts and Coronets becomes a duke after killing off the eight people ahead of him, all of whom are played by Alec Guinness.
 * This is the entire plot of King Ralph. In that case, it involves the British Royal Family, and the person selected had no idea he was in line in the first place.
 * Shanghai Knights had an noble who was way, way far down the line of succession hatch a conspiracy to kill everybody ahead of him so he could ascend to the throne.


 * By Dawn's Early Light, an adaptation of Trinity's Child below.
 * Eagle Eye.
 * Mars Attacks!!. After the death of all top U.S. officials, at the end of the movie the President's daughter is apparently in charge of the government. Yes, it's a comedy.
 * But then, the only thing she's actually seen doing is presenting Richie with his medal, probably because Richie isn't really the type for a more formal ceremony. One assumes that someone else would be in charge.
 * That would be the mariachi band leader.
 * XXX 2: State of the Union. The Secretary of Defense attempts a coup that will wipe out key members of the government during the President's State of the Union address, leaving him in charge.
 * In Johnny English, villain Pascal Sauvage gets the Queen to abdicate so that he, a descendant of William the Conqueror, is named her successor.
 * Also, Johnny English himself only gets promoted to field agent because every single other field agent gets killed.
 * In Stardust, after the king dies, his sons kill each other so the remaining one can take the throne. They all end up dead, but Tristan's mother reveals that she is the king's only daughter, meaning that Tristan is the only surviving male heir and thus, the new king.
 * This is the fuel that drives the plot of Mr. Deeds (and by that virtue Mr. Deeds Goes to Town). Upon the untimely freezing-at-the-top-of-Everest of Preston Blake, the entirety of Blake Media and its vast fortune now belongs to the only known relative of Blake, Longfellow Deeds (played by Adam Sandler, or by Gary Cooper in the original film), a greeting card writer and pizza shop owner from a small town in New Hampshire. This is played twice, being that
 * Michael Corleone was Unexpected Successor to the Corleone clan in The Godfather.
 * Used as a one-off joke/Take That at the end of My Fellow Americans. The President and Vice President both resign. Former Presidents Kramer and Douglas (the protagonists) realize that that means the Speaker of the House is next in line, and Douglas remarks "Oh no, not him!". At the time of filming, the speaker was Newt Gingrich.
 * Kull The Conqueror starts with the eponymous barbarian (played by Kevin Sorbo) being denied in joining the king's army, as all of them are noble-born. Then the king goes berserk and murders most of his successors before being mortally wounded by Kull. While the captain of the guard and a nobleman bicker over who should claim the crown, the king decides that all three should be punished and gives the crown to Kull before dying (believing that Kull will hate being king). The priesthood approves, and, suddenly, the captain of the guard must bow down before a barbarian he has just rejected from the army.

Literature

 * Ermintrude Daphne from Nation by Terry Pratchett spends her childhood being told by her grandmother that she needs to learn how to behave like a lady; if 138 people die, her father will become King of Great Britain and Ireland. One outbreak of Russian Influenza later...
 * When the Gentlemen of Last Resort come with the news that her father is now king, she asks nervously if her grandmother had done anything...silly.
 * In the novel Alas, Babylon the Secretary of Health, Education, and Welfare inherits the presidency after nuclear war.
 * Also briefly mentioned in Gaiman's story, "We Can Get Them For You Wholesale".
 * Deep Six by Clive Cussler. The Secretary of State becomes the Acting President.
 * Empire by Orson Scott Card. Terrorists assassinated the President, Vice President, and the entire Cabinet, so the Speaker of the House was president until the next elections.
 * Trinity's Child by William Prochnau. As the result of a nuclear attack on the U.S., the Secretary of the Interior assumes the Presidency. Later, the President is found to still be alive, and a power struggle ensues.
 * Warday by Whitley Strieber and James Kunetka. After a nuclear attack, the Deputy Secretary of the Treasury (the highest ranking remaining official) becomes President.
 * Worldwar: Striking the Balance by Harry Turtledove. In an alternate history, Secretary of State Cordell Hull assumes the Presidency after Franklin Delano Roosevelt's death.
 * This is several years after Washington, D.C., is nuked by the Race near the start of the invasion, eliminating much of the succession line, including the VP.
 * A sort-of case in the Colonization series, taking place 20 years after Worldwar with Vyacheslav Molotov, who is the leader of the Soviet Union after Stalin's death. In Real Life, Molotov never wanted to lead the country, although he was temporarily a member of a triumvirate. Additionally, Heinrich Himmler is the new führer of the Greater German Reich after Hitler's death, although that is not unexpected.
 * Merlin, the protagonist of Roger Zelazny's second Amber series, ends the series by becoming the ruler of Chaos after everyone ahead of him kills each other off.
 * Merlin's uncle Random, youngest of King Oberon's children, unexpectedly inherits the throne via Deus Ex Machina (the Unicorn) in the final book of the first series.
 * In George R. R. Martin's prequels to A Song of Ice and Fire, the Dunk & Egg stories, a little boy named Egg is a major character. In the main novels, we learn he became Aegon Targaryen, the Fifth of His Name, King of the Andals and the Rhoynar and the First Men, styled Aegon the Unlikely for assuming a throne no one expected he'd get as he was the youngest son of a king's youngest brother. He proved an excellent ruler though, and given his older brothers, the Seven Kingdoms came out lucky that time. It's generally accepted that he was the last decent one.
 * Aemon Tarygaryen, one of his older brothers, survives to the main stories as an old, old man who took the Maester's Chains and went to the Wall as a young man. He dies at 102 asking in his delirium for Egg. He tells a story to a young Jon Snow about how, many years ago, before Egg was crowned King, the nobles and the Maesters asked him to renounce his vows and take the throne. Egg was only crowned because Aemon took his vows seriously.
 * King Robert Baratheon hails from a cadet branch of the Targaryen line, so his succession was just as unlikely. By all accounts he graciously accepted the position of King after he smashed the Targaryen dynasty, but before he could even be crowned there were a few tense moments where anyone from Ned Stark, Tywin Lannister, and even Jaime Lannister could have proclaimed themselves king, but didn't. Then there's also the unfortunate fact the legitimate successor that he killed (Rhaegar Targaryen) would have been a perfectly decent king compared to his father Aerys.
 * Later, in the North, Eddard Stark, Lord of Winterfell and Warden of the North at the start of the books, has five legitimate children (three boys, two girls), an illegitimate son and a younger brother. Both brother and bastard son have taken the vows as members of the Night Watch and are therefore removed from succession (Jon Snow, the illegitimate son couldn't have inherited anyway).
 * Daenerys Targaryen has taken to calling herself "The Queen Across the Water", and has assumed the mantle of the Targaryen heir apparent in exile after her older brothers and nephew are each killed before they could assume the throne from their father/grandfather, King Ayres Targaryen II. As Dany hasn't reclaimed the realm of Westeros yet (the possibility of it even happening is still very much in doubt) she's more of an Unexpected Successor-in waiting.
 * In A Dance with Dragons, we learn that
 * In I, Claudius, Caligula is dead, and some of the conspirators who killed him are killing every member of Caligula's family they can find. The last thing they expect is the Praetorian Guard to declare Caligula's harmless uncle Claudius emperor.
 * Prince Roger of the Prince Roger series is behind two siblings and his brother's kids in line for the throne. Then while he's marooned on a backwards planet By the end of the series, he takes the throne.
 * The Balitang family in Tamora Pierce's Daughter of the Lioness books- the line of succession in the Copper Isles goes as follows (based on the beginning of Trickster's Choice): Oron, Hazarin, Dunevon, Mequen Balitang. When  the fact that Mequen's in line to the throne becomes a lot more important. When
 * Jack Ryan in the Tom Clancy novels. He's only Vice President, meaning he is directly next in line, but a) he was explicitly told, and had it as a condition of accepting the post, that he would not have to do much of anything, b) he was not expected by anyone, readers included, to ever run the country, c) he was only going to be VP for a few months, an unlikely time frame for an assassination, and d) he was only VP for an hour, tops, before becoming President.
 * In the The Riftwar Cycle series, Arutha starts the series as the lowest member of the royal family and was expected to become a minor duke, but in the first book alone so many nobels get killed off he becomes the second most powerful man in the Kingdom. In fact this trope is ever present in the seriesas nobilities die and need to be replaced at a very fast rate.
 * In Garth Nix's Sabriel, the Old Kingdom has been without the royal bloodline to protect it for a hundred years, since Kerrigor murdered his mother and sisters and used their blood to break the great Charter-stones. Politically and religiously beheaded, the only thing the Kingdom has on its side is that all other countries leave it alone on principle. Then Sabriel rescues a young guardsman who'd been transformed into the prow of a ship, and they go questing together. It turns out he was the illegitimate son of the Queen, an utterly improbable inheritor, and the last living prince.
 * He becomes King. He's good at it. He keeps the fool's-name, Touchstone, that he took on being awakened in memory of his total failure, but King Touchstone is not treated as a joke. Also, he and Sabriel spend all their time working incredibly hard. Not fun. For an extra plus, though, rejoining their bloodlines somehow enabled the resurrection of the extinct Wallmakers, and Prince Sameth does Magitek.
 * In The Belgariad and related works:
 * Garion himself is a rather Unexpected Successor, as most of the world thought the Rivan line had perished. Polgara and Belgarath didn't even bother filling Garion himself in (but the audience knows).
 * In the prequel Polgara the Sorceress, the whole hidden Rivan line of succession Polgara spends half her life protecting flows from a youngest grandson, the sole survivor of a slaughtered royal family. Subverted in that he doesn't actually get to rule; they need to hide him and his descendants.
 * Inferno by Roger Mac Bride Allen. Political wags had joked that nobody would assassinate the governor of the planet, Chanto Grieg, because the designated successor to the office was widely expected to be the equivalent of the Speaker of the House in the USA: President of the Legislative Council Shelabas Quellam, a man widely known to be good-natured but completely ill-suited to the office of Governor were he to ascend to the office. But Governor Chanto Grieg of the planet Inferno is indeed assassinated. Imagine the look of surprise on everybody's face when Chanto Grieg's will is officially read by his lawyer. The next Governor of Inferno is actually . Suffice it to say that Grieg knew what he was doing.
 * In Dirge for Prester John It's debated whether or not John should be allowed to take part in the Abir when he knows so little of Pentexore's ways, because what if he becomes someone important? And then he's the king.

Live Action TV

 * Laura Roslin from Battlestar Galactica Reimagined, who as the Secretary of Education was 43rd in line of succession. She became President after the Cylons' nuclear attack killed everyone higher-ranking than her.
 * She was actually asked to resign her position by the President a few days earlier, after going against his wishes when dealing with a teacher's strike. Her resignation would be have been made official when she got back to Caprica from the Galactica.
 * Part of the aliens' plan in the Doctor Who episode "Aliens in London" was to kill off enough of the British government to make the MP who they'd replaced with an alien into Prime Minister.
 * Why didn't they just impersonate the Prime Minister? He was too skinny to fit inside.
 * Jericho. Nuclear attacks leave the Secretary of Health and Human Services as the highest surviving official. Some people do not agree...
 * A sketch in one of Spike Milligan's shows had a Britain devastated after a nuclear war where the national anthem was "God Bless Mrs Ethel Stokes".
 * During a kidnapping crisis in The West Wing The President temporally steps down, and since there is no Vice-President at that moment, he appoints the Speaker of The House as the new commander-in-chief. Bear in mind, in this scenario a Democrat is passing the power to a Republican.
 * Brazilian mini-series O Brado Retumbante tells the story of an honest congressman who is elected Speaker of the House with the aid of a corrupt senator looking forward to use him as a puppet, when suddenly becomes the leader of the nation when both the President and the Vice President suddenly die in a helicopter crash.

Newspaper Comics

 * An arc in Doonesbury had the characters (during the Reagan administration) playing a computerized war game. Overreaction to a "Soviet provocation" results in nuclear war. In one of the last strips, the line of succession has resulted in Secretary of the Interior James Watt being President—with nothing much left to preside over. Watt's perceived anti-environmentalism is referenced with the remark, "Guess he got rid of all those trees."

Video Games
"After a polite discussion with local rivals, Vabok Praisehall has claimed the position of queen of The Feral Anvil."
 * King Airyglyph in Star Ocean: Till the End of Time, who was known as Airyglyph the Unlikely.
 * Ashnard in Fire Emblem: Path of Radiance killed his father and everyone who was ahead of him in order to get the throne of Daein, starting from fairly far down the line of succession. The sequel claims he killed everyone else of by . This is unpopular among the fanbase as it seems to have been thrown in there to demonstrate the power of the (which was never mentioned in the first game), and takes away from Ashnard's personality of loving to kill people firsthand.
 * In Betrayal at Krondor, this is part of Gorath's backstory. He becomes chieftain at the age of twelve when his tribe very nearly gets massacred, including the former chieftain, his father. Oh, and a dark elf like him would otherwise be expected to spend at least a century or two getting prepped for the position and would need to have lots of accomplishments to his name before being considered even marginally eligible.
 * GDI Director Redmond Boyle in Command & Conquer 3 was originally the the GDI Treasurer, and was the only ranking member of GDI government not on the Philadelphia when it was destroyed. However, it is revealed in the Expansion Pack that Kane purposely manipulated events so that Boyle would become Director.
 * This can happen to the player in Shogun: Total War, and possibly its sequels. Because of the way that births of heirs, aging, and succession are modelled, if you fail to manage your family properly, odd stuff can happen. Only the player's current character can die of old age or sire children, and the chance is random on any turn with penalties for age. For instance, the 90-year-old uncle of the current Daimyo, immune to age since he's not the head of the family, could be the only one left if the Daimyo's sons all die in battle or assassinations. He succeeds the throne when his 60-some year old nephew dies, and then himself dies the next turn on a random old-age roll buoyed up by 30 years of penalties, ending the game.
 * In later games in the Total War series, the game doesn't end, your country "merely" goes into civil war as any general with a drop of royal blood tries to claim the throne. The player is allowed to pick one of the claimants to make the de facto legitimate heir, and everyone else gets treated as a Rebel faction by the game. Players at risk of this have been known to marry a princess to their best general, which gives him a claim to the throne too. After all, if you're gonna have to fight your own guys, might as well do it with your best commander and biggest army. In in-game terms, this can result in a minor lord, knight, or even commoner being vaulted onto the throne.
 * Peony in Tales of the Abyss. He's the illegitimate son of the emperor, when his half brothers all die he ends up being shipped back to the capitol for Cram School a la governing, instead of going down a cadet line, or some other noble house. In this case, "unexpected" only describes an outside perspective, though: because The Score (the prophecy that controls the world's fate) is a completely accurate prediction of the future, Peony's father had been told that he would eventually ascend to the throne. That was why Peony was sent away from the court intrigues to live incognito, where he ran off from his guards to play with commoner children.
 * Tactics Ogre is in love with this trope. The previous monarch, King Dorgalia, was a commoner who managed to become the first king of Valeria by defeating his biggest rival, King Roderick, in the middle of a bloody ethnic civil war. Dorgalia has a legimate son with a Bacrum noblewoman, but he dies at a young age and so does the Queen. This leaves Valeria in yet another civil war, which each one of the ethnic groups being led by a different pretender, all trying to become King. Ultimately it turns out that Catiua, the protagonist Denim's adopted sister, is the unknown bastard daughter of King Dorgalia, and thus the rightful Queen. Depending upon the ending, either she can be the Unexpected Successor, or it can be Denim himself if she gets killed. Denim's military might is really the only thing that holds together either crowning. Literally every other rival is dead by then.
 * A Succession Crisis in Final Fantasy Tactics leaves nearly everyone with a semi-legitimate claim to the throne either dead or otherwise disposed of, paving the way for Delita, a once-poor stable boy who worked his way up through the military via masterful Xanatos Speed Chess, to take the throne of Ivalice by marrying Ovelia, the only surviving claimant just before her coronation. Ovelia's ascension was just as unlikely and just as masterfully orchestrated, only by powers beyond her own control. She too was a commoner who was switched at birth with the real Princess Ovelia, who had already died, and raised as the ailing king's younger sister. She was then used as a political pawn by Duke Goltana, who intended to place her on the throne, assume power as her regent, and then have her executed. Ultimately, Goltana is betrayed and murdered (by Delita), even though his faction wins the War of the Lions, so Ovelia ascends to the throne as a proper queen ... or would have, had Delita not married her and become king by default..
 * Final Fantasy XII sees command of the Imperial army fall into the lap of Judge Zargabaath, the rarely seen low man on the Judge Magistrate totem pole simply because all the other Judges are either dead or have jumped ship.
 * In the Neverwinter Nights module "The Bastard of Kosigan", your character is the illegitimate son of the present Count's younger brother; the present Count has two legitimate and one illegitimate son, and the elder legitimate son has a wife and son. Over the course of the game, all five people in line ahead of you (your father is already dead) get killed off by you/Alex/each other/French assassins, leaving your character with the best claim. The French plot initially involved killing you off as well, which would have left a French general with the title by virtue of being of a branch of the family that diverged several centuries ago.
 * The Elder Scrolls IV: Oblivion has a good example of this; The Emperor Uriel Septim was assassinated, along with all four of his sons. However, he has an unknown illegitimate son - Martin Septim - who becomes Emperor after you deliver the Requisite Royal Regalia... and then deliver Martin. Though
 * The player character in the Mechwarrior reboot is fourth in line of succession for his family, the planetary rulers, and ends up becoming a lazy, party-loving Jerkass as a result. Then the rest of his family is assassinated and he suddenly finds himself in the front lines...
 * Happens twice in the same country in Dragon Quest VIII. King Clavius of Argonia only became the king because his elder brother deserted the country. Then his useless son Prince Charmles is expected to be his heir, until
 * In Dwarf Fortress, after one of nobility upgrades succession works, in that titles are inherited by someone else upon death of the previous holder... So if the King of your dwarven civilization is killed by raiders or a megabeast, another dwarf becomes the king. And they have relatives all over the realm. Including outposts - which means that you may get an announcement like this (quite appropriately, in purple):

Web Comics

 * Its Walky: Walky's mother becomes Big Boss after the previous one is killed and everyone ahead of her on the SEMME hierarchy is too old, crazy, or dead to take over.
 * As his Start of Darkness reveals, Redcloak from Order of the Stick was the newest acolyte in the hierarchy of the Dark One's priesthood, and became the High Priest because

Western Animation

 * Used as a gag in the Animaniacs episode set in Anvilania. Yakko arrives to take over the throne, and sings a very complicated song explaining where he fits in the line of succession (extremely low), which ends with him noting sadly that everyone named in the song is dead, leaving him King.
 * Avatar: The Last Airbender:
 * Azula becomes very briefly Fire Lord in the end of Season Three (though the coronation gets interrupted just before she's actually crowned). As far as we are given to understand their succession scheme, she is the lowest in the entire line of Sozin: the youngest and a girl, though since she's also the only girl born into the family in the period we see we don't find out how much that impacts her eligibility. It doesn't seem and isn't treated as odd because she was the only one left anyway. But ten years earlier? The succession evidently ran Azulon --> Iroh --> Lu Ten --> Ozai --> Zuko --> Azula. Sixth and last in line. After this we have to go to pretty distant cousins who never appear, given Sozin and Azulon were apparently only children.
 * Zuko's own accession was both fairly inevitable and actually kind of improbable, although he was technically Crown Prince for at least five years before gaining the throne.
 * Ozai only got the spot by luck, opportunism, and backstabbing. Oh, and obviously flexible rules of primogeniture.

Real Life

 * Further down The House of Windsor, George VI was not expecting to become King, going for a military career. Then his brother abdicated.
 * While it has never actually come into play beyond the well-known examples of Vice Presidents taking over for their dead Presidents, the US government has a very specific line of Presidential succession that specifies who would become President in the event of the inability of multiple people in line to serve. This did come up among pundits in the aftermath of the 1981 assassination attempt against Ronald Reagan, where Secretary of State Al Haig stated to the press, "Pending the arrival of the Vice-President, I'm in charge here at the White House." While he meant that he was just running the White House and Presidential offices until VP George H.W. Bush could return from his vacation in Maine, the pundits all ignored the context and treated it as if Haig was trying to stage a coup. (Until Bush could be contacted, the acting President was technically Speaker of the House Tip O'Neill, though his primary responsibility was command over US nuclear forces until Bush could be contacted.) Because this line of succession also puts a hard limit on how far down the governmental food chain the Presidency can fall, the Secret Service makes certain that, at any event where the entire succession would be expected to appear (such as a State of the Union Address), one person in the line of succession is not in attendance, instead being kept at a safe house far enough away that no single plausible catastrophe could kill him along with the rest of the succession.
 * The "Designated Survivor" rule has been unofficially extended by Congress to include at least one Senator and one Representative, so that in the event of a decapitation strike there would also be successors to the roles of Senate President Pro Tempore and Speaker of the House.
 * The aforementioned line of succession is somewhat random. Currently, if something happens to Obama, the office passes first to Vice-President Biden, then Speaker Boehner. Okay so far. If something happens to all three, as of December 2011, the man the United States would turn to is...Daniel Inouye, the 85 year-old Senator from Hawaii who holds his position solely by virtue of being the longest-serving member of the majority party. (The spot was held by the even older Robert Byrd until his recent passing.) The Secretary of Homeland Security, who you'd think ought to be in charge after a decapitation strike, is last in the list. This is because the rest of the line of succession is determined by the age of the office, and since the Department of Homeland Security is the newest department, the Secretary of Homeland Security is last in line, while the fourth in line is the Secretary of State. (Besides, one imagines they'd be rather busy at that moment.)
 * Gerald Ford, the only truly unelected President Of The United States. All arguments about contested elections aside (and there are plenty), Ford was never elected to the vice presidency (he was appointed after Vice President Spiro Agnew resigned), then became president after Richard Nixon resigned. Then he lost his first election. All Gerald wanted to do was be Speaker of the House.
 * Henry VII would not have been expected to be King. He was the half-nephew of the King at the time of his birth, Henry VI, but his claim actually came through his mother's side and was fairly weak. (She came from the line of a legitimized bastard.) Still, he was next in line after Henry VI's son Edward. Then Edward IV took over, and he had two sons of his own, not to mention two brothers. No one thought Henry Tudor could beat those odds. Possibly not a straight example since Henry himself forced his succession via a battle, but he was still an unlikely pretender.

Of course, Henry VI's claim to the throne already was a bit iffy as his grandfather Henry IV had deposed (and probably ordered the murder of) Richard II and pushed aside the legitimate successor to become king. Henry VII made sure of the throne by marrying the most plausible other successor, Elizabeth of York.
 * While we're discussing the Tudors, in 1547, when Henry VIII died, Mary Stuart, Queen of Scots was 4th in line for the English Throne. With three healthy legitimate children alive, it was considered very unlikely the Stuarts would ever get a hold of the throne. All three of his legitimate heirs assumed the English throne at some point, and all three died childless. 56 years after the death of Henry VIII, James VI Stuart of Scotland assumed the English throne as James I.
 * And then, the successors to the Stuarts, The House of Hanover. Due to the exclusion of Catholics from the succession by the Act of Settlement 1701 and the death of Queen Anne's children, the Prince of the German Electorate of Brunswick-Luneburg (also known as Hanover) was more or less handed the throne of Great Britain out of nowhere. This is why George I and George II spoke little to no English: George I and II were 41 and 18, respectively, when the Act of Settlement passed, making them second and third in line (after George I's mother Sophia), and were 54 and 31 when the throne passed to their house. This led to the development of government by the King's ministers rather than the monarch himself, and by the end of George I's reign, the general system used in Britain today had been developed under the guidance of the (unofficial) Prime Minister Sir Robert Walpole. So, indirectly, we have this trope to thank for the modern system of parliamentary democracy—used in some form by the vast majority of democratic states in existence today.
 * The possibly greatest subversion of the trope was Ramiro II of Aragon. The fourth son of King Sancho, he wasn't expected to inherit or hold a political position at all and became a priest. However, all three of his elder brothers died without issue, two of them after having seized the crown. He was then literally taken from his abbey, given a Papal permission to abandon his vows so he could guarantee the survival of the dynasty and crowned. He complied, married, had a daughter, abdicated to her and had her married when she was 1 year old. With his deed accomplished in record time, he took the vows again and went back to his abbey.
 * Invoked in the later two-thirds of the Qing dynasty of the Imperial China. Yongzheng emperor had his legitimacy consistently questioned due to the Succession Crisis in 1710-1720s (in which his father deposed the crown prince and did not set up any up to his death), and decided that while the monarch have the power to nominate any successor, there should not be any expectation for anyone to be one. The modus operandi is thus: every moment the emperor was alive, he was required to keep a succession will sealed and hidden somewhere in the palace, and would only be opened at the time the said emperor passed away. As a result, no heir would be publicly named, and while the emperor's personal preference may be apparent, it would never be any indication of who would be succeeding.