During the battle royale in Yu-Gi-Oh! East Academy, Marcus screws up his entire strategy twice by misreading his own cards.
Destroy The Godmodder has many such events, the first notable one being the reapers. A massive army of them gets summoned, what happens? The godmodder mind controls them into changing sides. It isn't the worst offender either.
It turns into this for the godmodder later on, and everyone gets a slice of the action with the Glitch.
The Virus in game 2. Seems like a great idea, it gives the A Gs a fair boost... and then gains self awareness. And decides it wants to kill everything.
In this sadly truncated Doctor Who-themed Mafia game, the Weeping Angels come off very badly. How badly? The only player to die was their most powerful player. Their attempt to kill one Genre Savvy player got busted because said player turned out to be the Doctor. Attempts to get in close with said player (who they didn't know was the Doctor) to bust open a potential good guy network killed the aforementioned most powerful player. Ouch.