Beginner's Luck

At any gambling casino, from Monte Carlo to Reno,
They'll tell you that a beginner comes out a winner.
Beginner fishing for flounder will catch a seventeen-pounder...
That's what I'd always heard, and always thought absurd,
But now -- I believe every word.

—Fred Astaire, "(I've Got) Beginner's Luck" (words by Ira Gershwin)

Sure, the Big Bad has slaughtered thousands of experienced soldiers and dozens of truly ace opponents, or perhaps has already taken over planets. That Rebel Alliance has no chance, and the Galactic Senate—doomed.

If you see a plucky kid who's never used your weapon of choice before, however, it's time to turn tail and run.

Plucky kids seem to have some innate link to Destiny, since they always seem to end up with the one power necessary to rule them all. Even worse, they tend to be savants at that thing, even when individuals around them have trained and worked for years only to be bested by a plucky kid within a few minutes.

Compare with Falling Into the Cockpit and Readings Are Off the Scale, although Beginner's Luck applies to a person rather than a machine.

Examples of Beginner's Luck include:

Anime and Manga

Subversion: Shinji from Neon Genesis Evangelion has a remarkable ability to sync with and pilot his Humongous Mecha, even compared to those who have trained for months and even been cloned with the purpose of doing so. He's also the only one to cause his Humongous Mecha to go berserk in the main series for significant amounts of time, including it moving when it has no power. However, this all has been carefully planned out in advance by the Powers That Be, and it rarely translates well when the situation calls for finesse.
Amuro from Mobile Suit Gundam drops into a Humongous Mecha and, without even having used the controls before, handily survives an attack by an enemy war veteran. Handwaved by letting Amuro read a manual for the Gundam—for a few minutes, anyway. Most main characters from the spin-off series do the same, although there are exceptions.
- Later subverted. While he had the advantage of a massively overpowered machine compared to his enemies early on, as Zeon threw superior machines at him he started having a lot more trouble despite new weapons and the required experience using them.
- Played with in Mobile Suit Gundam ZZ. Despite being Newtypes, none of the pilots can even walk properly on their first sortie. Judeau managed to drive off his first enemy in his first sortie, mostly through confusion on his opponent's part and...well, luck.
Kasuki Yotsuga from Parallel Trouble Adventure Dual discovers his incredible talent for piloting Core Robots when he's accidentally sealed into one while trying to rescue its wounded pilot.
Lyrical Nanoha. As a kid fumbling around with her staff for the first time, she destroys a monster within minutes, she saves multiple universes multiple times at the age of nine, completely shattering the learning curve in the process, and years later as a fully trained battlemage, she's not only a literal ace (well, "A's") but has to have limiter upon limiter attached to herself and her staff at all times so as not to godmode her enemies to death in seconds.
- The in-story explanation for the limiters is that a single unit is not allowed to have that many high-level mages in it, for fear of mutiny.
Misaki in Angelic Layer is known as the "Miracle Rookie", beating her opponents with the simple power of Power Copying and friend-magnetism when she's barely picked up an Angel. An interesting example because not only is this a game rather than world-saving heroics, it's just that, a game, not Serious Business - at least in the manga. (Even in the anime, Misaki doesn't treat it nearly as seriously as others do.)
Like many anime before it, Bleach's the main character starts as a simple human who can see ghosts. Turns out he not only has enough raw power to break magic, but then manifests a Giant Sword of Doom. For the first few arcs, he continuously manages to not only gain powers and/or abilities that other people have to work centuries to gain IN DAYS, but then continues to show that he is better at it.
- He also has no talent whatsoever for performing hakudo spells or special moves though. While his spiritual pressure is tremendous, he's just as tremendously limited in its use. He only has one special move, and even his bankai is just a massive speed boost.
From Saki, Senoo Kaori from Tsuruga Academy is made of this trope. She's a complete noob, even going so far as to visually (and obviously) separate her tiles according to her winning hand. But she gets lucky many times, to the point where Yumi forbade her from playing Mahjong between the team competition and the individual so she could keep her luck. The result: First, in the team tourney, she slams Kiyosumi's Mako with a Suuankou (4 concealed triples, a rare Yakuman hand worth max points). Then she goes on to win her first individuals match by scoring a Kokushi Musou (aka Thirteen Orphans, another rare Yakuman hand) off of Ryuumonbuchi's Touka. Finally, just for fun, she drops a Ryuu-issou (All Greens) Yakuman in practice. Of course, she never calls out her hand correctly, because she doesn't even know which combo is which (each of her 'victims' had to tell her which hand she really won with).
Yuuri in Kyo Kara Maoh commits this trope when he manages to defeat Wolfram in a sword-fight despite having no sword training at all. He then proceeds to defeat him using magic too, with no training or even knowledge of his own ability, even though everyone else he faces have a lifetime of training and effort. To be fair, though, he almost never uses magic outside of his Super Mode, which comes complete with a Split Personality that does know the ins and outs of spellcraft very well.
In Pokémon, Ash's very first matchup in the Indigo League, he decided to use a Pokémon that has had no experience in battling before: his Krabby. Krabby proceeded to evolve into Kingler and swiftly gives Ash his first win in the tournament. Brought up again in "Hello Pummelo", where Ash chooses another Pokémon he has never used before, his Tauros, in his lineup for the championship match.
- In the same vein in Best Wishes Ash's new rival Trip easily beats Ash's Pikachu with his Snivy. He lampshades how easy it was to beat Pikachu but the audience knows that Pikachu wasn't up for a battle because earlier in the episode, Pikachu was zapped by Zekrom and couldn't use his electric or speed attacks severely making Pikachu having a huge handicap. While Trip doesn't believe Ash when Ash explains that in their next battle, Ash clearly tells Trip that he got lucky their first battle.

Film

Anakin and Luke Skywalker in Star Wars: Anakin flies a pod racer pretty much perfectly, even though full grown aliens crash and die all the time in the races. He then promptly gets into a starfighter that he's never seen before in his life and manages to fly it well enough to not only kick ass, but blow up a massive spaceship. Later in his life, while rescuing the Chancellor, he successfully lands planetside a ship whose controls he's never seen before, which is not designed for atmospheric , not designed for human pilots, has broken gravity generators, is missing about half its mass, and is on fire. He sticks the landing (right outside the Jedi temple, having a whole planet of landing sites to choose from) and walks away from the smoking wreckage. Luke, meanwhile, has at least some excuse. The Incom T-65 X-wing setup is almost a carbon copy of the Skyhopper's, which is a small air speeder that Luke practiced with throughout the majority of his life on Tattooine, allowing Luke to adjust fairly quickly.
- Not sure Anakin qualifies with Podracing-The movie makes it pretty clear he's been doing it for a while and the race he wins to free himself is, forget winning, one of the first races he's ever finished!

A rather humorous Double Subversion occurs in True Lies. Helen Tasker, a civilian held hostage by the Big Bad Arabian terrorists, picks up a fully automatic submachinegun for the first time... and promptly fumbles it. However, in the act of fumbling, the gun bounces down the stairs in slow motion, randomly firing and killing all but one of the terrorists.
- This is a more extreme version of Susan Calvin successfully shooting a machine gun with her eyes closed in I, Robot.
Played especially straight in The Karate Kid. Daniel stumbles on Mr. Miyagi attempting to catch flies with a pair of chopsticks ("Man who catch fly with chopstick - accomplish ANYTHING.") While Mr. Miyagi tries and fails throughout the scene, Daniel manages it within seconds of attempting it, prompting a disgruntled "You beginner luck," from Miyagi.

Literature

The Wheel of Time. In the first book, Rand gets a blademaster's sword and starts learning how to use it. In the second book, he kills a real blademaster (one who earned a blademaster's sword by becoming a blademaster) in a fair fight.
In Robert Asprin's "Little Myth Marker", Skeeve (who has played precisely one game before this) plays Dragon Poker against the top professional player in the multi-verse, the Sen-Sen Ante Kid. Skeeve explicitly invokes this trope, stating immediately before betting his entire pot on the first hand that in a long term game he had no chance at all, but in a single hand he had a 50-50 chance of winning. He, of course, won.
In Gordon Korman's Beware the Fish, Bruno and Boots slipped a cold remedy created by school genius Elmer Drimsdale into the sports coach's energy drink. It reacted badly with the citric acid and mimicked intoxication, and the only way Bruno, Boots and Elmer could keep the coach from wandering over to the girls' school across the road was by agreeing to play poker for toothpicks with him. Elmer won all the toothpicks within the first half-hour, which Bruno termed "Beginner's luck."

Tropes

Inherent to the Sixth Ranger trope, often with The Worf Effect later.

Video Games

Possibly inspired by all the above examples of Humongous Mecha pilots, the main character in Zone of the Enders is an ordinary 14-year-old boy, who literally stumbles into the cockpit of the super-advanced Jehuty while trying to hide from an attack on the colony. Literally within minutes of entering this robot, he goes toe-to-toe with The Dragon, a psychopathic, suicidal veteran of countless battles...
- This seems somewhat justified after playing the sequel, as after seeing what Jehuty is capable of with a trained pilot in control, we realize exactly how little of the mecha's capabilities its first young pilot was able to make use of. It really was using only its most basic features, assisted by the AI; it wasn't just a handwave after all.
- Cage Midwell in Zone Of The Enders: the Fist of Mars similarly ends up fighting several battles when he ends up in control of a Super Prototype. In both cases, it's somewhat justified by the fact that the Orbital Frames in question also come equipped with powerful A.I.s who can help them in combat.
In Warhammer 40,000: Fire Warrior, the main character, Kais, tears through an enemy base, takes down a helicopter(equivalent) with nothing but a pair of rifles, rescues the Ethereal, almost singlehandedly stymies a boarding attempt on his ship, fights off entire squads of Space Marines, fights off Corrupted Space Marines, takes down a Humongous Mecha, and even destroys a Greater Daemon... all during his first battle.
- However he does go completely insane after this.
- In the novel you find out that the Blood God Khorne was so impressed with Kais' bloodbath that he started backing the little blue guy.
Gordon Freeman (of the Half-Life series). Who knew scientists knew how to operate firearms?
Justified in Knights of the Old Republic, where, near at the end of the game, you discover that you are Darth Revan with an almost destroyed memory, and thus your skills were there, you just had to awake them once again
The Nasuverse has several examples. Fate Stay Night gives us Shirou, who's been admittedly training for 8 years so hard it could nearly kill him... except he does it wrong and therefore it was pointless. By the end of the route he's generally taking on Servants 1v1 and winning. Shiki in Tsukihime is even more ridiculous, killing the 1000 year old Nero Chaos and 800 year old Roa, both of whom were supposed to be unkillable. With a small pocket knife, the only weapon he has any proficiency for at all.
- Shiki is a bit of a strange case. He has a power that can kill anything including abstract concepts such as invincibility itself, as well as a Super-Powered Evil Side that is vastly more competent than he is and seems to be somewhat more than human.
In Tokimeki Memorial 2 Substories: Dancing Summer Vacation, Kaori Yae manages to get a perfect score on her first try at Dance Dance Revolution. The justification given by the storyline is that she has an innate sense of rythmn, and is in top-shape physical condition due to being a sportswoman.
Final Fantasy has an enemy called Tonberry. Tonberry's Karma attack does damage related to the number of enemies its target has killed. This is usually a One Hit KO, but not if the character hasn't killed that many enemies but just survived on Leaked Experience.
Kentarou in the Rance backstory manages to defeat several demons and even the Demon King Gi. Of course, that's because said Demon King didn't feel like living once he gave the job to Kentarou's girlfriend, Miki.

Web Comics

Seems to be the explanation for the successes of, if not Scarlett, then at least for Harold and Clancy in Plus EV.

Western Animation

In Storm Hawks, The Dark Ace has already killed several experienced Storm Hawks, but the leader of the young, inexperienced group beats him. Multiple times.

Real Life

Often happens in Poker, since beginners almost always don't know anything about poker strategy and play as they like. So, they're a) hard to estimate and b) also hard to bluff. Of course, poker is still partly luck-based, so any beginner has a chance to beat the best poker players in the world. (If you're playing against a single opponent without a betting limit, simply betting all your chips immediately at the beginning of every hand - without even looking at your cards - will give you about a one in three chance to win a match against anyone.)
- There are some known strategies that are very good at beating beginning poker players. They don't work on experts, who know about them and what their flaws are, but techniques that good players use to get small advantages against each another generally work on the assumption that their opponent isn't an idiot and will react rationally to the information you reveal as you play. When playing against an actual idiot, they can be useless and even counterproductive (for example, there's no point in trying to bluff someone who never folds), but if you know your opponent is an idiot, you can use an anti-beginner strategy to take their chips rather quickly.

"The only two people you should never play poker with are the best in the world and the worst in the world."

Wilfrid May narrowly escaped being shot down by the Red Baron on what would be the latter's final flight because he was still a novice fighter pilot and Von Richthofen couldn't anticipate May's erratic movements.
The above two examples fall under an old military Murphy's Law corollary: "Professional soldiers are predictable, but the world is full of amateurs."
When it comes to playing a video game based on fighting, such as Street Fighter, the more experienced player will know all the tricks to every move on every character while someone with little to no experience in the fighting game genre may resort to Button Mashing. The newbie player may actually win a few rounds against an experienced player this way just because they managed to mash up enough buttons to pull off a series of moves to stop their opponent while the experienced player is frustrated since they can't find an opening to attack in or block against all the spammed attacks. Not all fighting games fall under this and may have counters against players who randomly mashes buttons.
When betting on horse racing, at least in the UK, the first bet someone places (who has never done so before) is known as a Virgin Bet. It is commonly held that a Virgin Bet is usually lucky.