Continuity mistake: When the twins are singing about Brad for the talent show, and Michael was playing the piano (you have to listen and watch), his hands are suppose to go across the keys but he is still doing the fake up-and-down-hand-move.
Continuity mistake: After we see the twins push Crater-Face into the pool, we see them a couple of scenes later after Michael reveals himself as the mystery rider and we notice that the twins are completely dry.
Continuity mistake: When Michael is doing his bike up and is spraying the cycle's gas tank, his hands are full of oil, but in between his fingers are clean. This shows that is is obvious someone has smeared the oil on, otherwise in between his fingers would be just as dirty. And his hands wouldn't be perfectly black, they would be streaked.
Continuity mistake: In the scene at the end when Johnny gets out of the raft, you see him walking up through the back of the crowd and in the next shot, he is throwing his costume off his back on the side of the pool, then cuts back to him coming between Michael and Stephanie.
Continuity mistake: The scene when Michael is jumping over the cop car and it cuts to a front shot of the jump, you can see Stephanie, Paulette and Rhonda watching him jump in the background, even though Rhonda had walked back inside the bowling alley with Goose and Davy before Michael reappeared from the chase.