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.
Visible crew/equipment: During the first encounter of the "Cool Rider" the police show up and he jumps over the car. In the rear shot, you can see the ramp on the ground under the police car that the stunt man used to jump the car.
Revealing mistake: When they're dancing on the bowling alleys, they couldn't be waving their bowling balls about as if they were made from foam - bowling balls are heavy!
Audio problem: When singing Reproduction, a boy sings "Oh I think I'm gonna throw up"; yet he sticks his finger down his throat and keeps it there until the line is over, never closing his mouth; how did he make the 'p' sound on 'up' without closing his lips together? The words are pronounced very clearly and the camera doesn't cut from his face until after the line is done.
Revealing mistake: During the luau when the motorcycles are attempting to jump over the pool, the cables that are stopping the cycles, allowing the riders to go flying into the pool, are visible.
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.
Plot hole: Nogerelli says to himself that the gap past Deadman's Curve must be 100 feet wide. How on earth was Michael meant to have made a jump of 100 feet (or even a few feet less) after both being slowed down by crashing through the wooden barriers and also by not having a ramp and therefore no upwards boost? He would have plummeted to the bottom.
Other mistake: Eugene is suspended above the stage with the box of paper leaves and snowflakes ready to throw them down at the correct moments; but when the Fall section of Girl For All Seasons is on he doesn't throw any leaves. Also, the amount of snow coming down over the stage at the Winter section is completely disproportionate to how widely and evenly Eugene could distribute it.
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.
Factual error: When the substitute teacher is teaching the children about reproduction, the Playboy magazine is from September 1962, but the year is supposed to be 1961.
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.
Other mistake: When Michael is sitting down singing the song "Charades," there is a window behind him. Dolores comes up to the window and waves for the Pink Ladies to come outside. First you see Paulette go by, then Sharon, and the third girl is obviously supposed to be Rhonda, as she is wearing a Pink Lady jacket. If you look at "Rhonda's" face, you can see it is not Alison Price, who portrays Rhonda throughout the rest of the movie. (01:22:00)
Other mistake: The movie starts in September 1961 and we are told that the talent show is in June. During the talent show there's a big banner saying 1961 when it should read 1962.