Sociopathic Hero Jobe of the Whateley Universe created a serum that would let him turn women into his dream girl - a drow hottie with incredible regeneration abilities. Yep, you guessed it. He accidentally received an injection. In addition, just to rub it in, the amazing regeneration means he can't find a way to turn back to his old self.
Power mimic Counterpoint has this happen to him more than once in the Whateley Universe. In a holographic simulation, the version of Counterpoint attacks Phase to steal Phase's powers; but Phase figures out how to use his density-changing powers to kill someone with density-changing powers. Counterpoint beats up Chaka to get Chaka's Ki-manipulation power; but Counterpoint can't use both yin and yang (he's a very masculine god of war) and so the built-up yin in his system nearly kills him before Chaka saves him. And then he tangles with Jobe...
In the College Humorspoof of Inglourious Basterds, Nazi colonel Hans Landa is an extreme stickler for grammar who becomes increasingly frustrated with Monsieur La Padite's grammatical errors and unwillingness to cooperate with him. Triumphant with his discovery that there are Jews hiding under La Padite's floorboards, Landa mistakenly utters a dangling participle and chooses to commit suicide out of shame.
Similarly, Rainbow Dash defeats Starscream by tricking his own guided missiles into hitting him instead of her.
A double dose in Dr. Wily vs. Dr. Eggman. Eggman uses Metal Sonic against his opponent, and Wily unleashes the Roboenza virus as a counter, which leads to Metal Sonic going on an indiscriminate rampage and both their demises.
Season 6 of Arby 'n' the Chief introduces the character, Justin. Revealed to be the mastermind behind the console banning hacks that Arbiter, Master Chief, Chaos Theosis and Trent Donnovich have been using in seasons 5 and 6. When Justin presents Trent with network crippling software, guess who Arbiter takes out first?
In one episode of Achievement Hunter's Things to Do in Minecraft, Geoff and Gavin create the Hop Till You Drop maze and takes glee in watching the other Achievement Hunters falling into the lava pit below. Flash forward to Let's Play Minecraft episode 44 and Geoff is the first one to find and mine the Tower of Pimps. A bad step causes Geoff to throw an Ender Pearl wildly. Where does it land? The lava pit, taking him out AND losing the Tower, forcing them to redo it over again.
In an episode of GO!, Geoff turns Ray's perfectionism against him as the latter is forced to choose a game using his primary Xbox Live account ("BrownMan" instead of his secondary "FluttershySucks"). The problem is is that Ray doesn't like having games left uncompleted on his account and thus he's left in an conundrum as he can't choose a game that he can't easily complete.
Another episode of GO! had Gavin win around because everyone else chose South Park: The Stick of Truth and were hit with a major update patch, allowing Gavin to coast to victory.
In chapter 5 of the Neopian Times story Revenge, during a Yooyuball game, Alex uses her sling to hurl a potion at Naqasa that will turn her into a hideous mutant Usul. However, Naqasa's brother Q'tai is also playing and throws his ball to hit the potion and send it flying in Alex's direction so that she gets drenched with it instead.
In Worm, Jack Slash suffers a Fate Worse than Death at the hands of Gray Boy, who he had cloned in the first place to replace the original. In addition, Golem, the hero that orchestrated these events, is in fact Theo Anders, who Jack had challenged to kill him two years previously.
In Noob Season 2 finale, Master Zen calls a Game Master on Arthéon and Sparadrap for possession of the hacked staff. When the Game Master shows up, he reveals that possession of hacked equipment no longer ban-worthy due the fact that the game authorities have realized that it was involuntary on the player's part. Master Zen's reaction is to take the cheated staff and try using it against Sparadrap and Arthéon, and act which is still forbidden. This scene is the equivalent of calling the police on someone for misconduct, having it transpire that the misconduct was involuntary, then having the person who called the police doing something arrest-worthy while the agent is still present.