A Chekov
When a guy in a hat is on the screen for five seconds, but then he becomes the evil mastermind in the final act. No one knows why, but it's always the worst twist.
That old man in the background turned out to be the villain. No one saw it coming.
The guy who sold me the pizza was the secret villain. Classic.
The janitor in the movie turned out to be the leader of the evil empire.