To flub something is to really mess it up. When a movie actor flubs his lines, the scene can be filmed again. But, if he's a stage actor, there's no way to fix the mistake.
Even a star basketball player flubs an easy layup sometimes, and the very best students occasionally flub a quiz. The mistake or blunder itself can also be called a flub: "The concert was terrible, full of flubs." The word's been around since the 1920's, possibly influenced by words like "fluff" and "flop," and definitely coined in the United States.