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.
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.
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!
Suggested correction: Bowling balls can weigh as little as 3 pounds.
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)
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.
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: 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: "We're Gonna Score Tonight" - 3:30 into the song, Johnnie rolls a ball down the lane and turns around, throws his arms in victory. In the very next shot, he is jumping from some height to the ground. There is also an obvious cut in the music at this point.
Continuity mistake: Dimucci is wearing grey trousers when they start singing Prowlin, but he has jeans on for the second half of the song.
Suggested correction: That's because it's gone from the night out with the gang to the next day at school.
I'm not sure about this "correction." Every other song in this film is done in real time, why would this particular song take place over 2 days? Since the rest of the T-Birds are in the same clothes, and Dimucci is wearing the same shirt, I would say it was a mistake that his pants changed.
They are all wearing different clothes except possibly Johnny. Goose goes from a flannel shirt to a blue T-shirt. Davey goes from a brown shirt to a yellow shirt. DiMucci goes from a red shirt to a blue/grey shirt. Johnny is wearing all black, but he does so in every scene except at the bowling alley. The start of the song is at night and the second part is at school, meaning they would have to be there during the school-day or just after when they would have access to the auditorium.
Continuity mistake: During "Score Tonight" as Johnny is singing 'hey Paulette...' we can see Goose lifting Dolores up onto his shoulders but in the next shot he is holding her piggyback.
Other mistake: During the bike chase at the bowling alley, when the line "don't know who you are" is sung, a guy on a car hood throws his body to the right as if he were pushed by an invisible hand but in the next shot he repeats the motion when a bike hits the car he's sitting on.
Suggested correction: I've watched this scene several times since this I saw this post and can confidently say that the guy was going along (dancing?) with the music on the first instance of his motion; he is in perfect timing with the line you referenced.
Continuity mistake: When the movie starts, two women raise a flag. The hands of the one on the left change positions between shots.
