Villain goes against a non main character thinking it is going to be an easy win. It isn't.
Yu-Gi-Oh: Marik vs Joey: Marik believes a duel against the only none Egyptian God weilder will be a easy win, and goes most of the duel toying with him. Turns out, if Joey didn't passed out, he would have won. Marik even goes as far as puking from the effort in the original anime.
Dragon Ball Z: Tien vs Cell: Cell has his full focus on absorbing Android 18. When he sees Tien trying to stop him, he brushes it off as a mere nuisance, only for Tien to actually stall him enough for Android 18 to escape.
Young Justice: One of the main themes of the show. Villains see them as children and mere sidekicks from the main heroes, so they mostly focus on how to deal with the main heroes in the first two seasons, either by mind control or sending them off planet. Turns out, when they have to step up to the plate, they are more than capable of handling themselves.