Continuity mistake: When Principal McGee does her announcement to all the students at the beginning of the film, you can see a flag of the United States at the far end of her desk, but when we see a close-up of the Principal, the flag is now right in front of her.
Continuity mistake: During the dance contest, when Kenickie and Cha-Cha are dancing around like crazy (when she puts her foot up on his shoulder), you can see Danny and Sandy in the background, standing on the sidelines. Given that they're the last couple on the floor at the end, they should still be out dancing at this point.
Continuity mistake: When Sonny enters the sports hall for the National Dance Off, he twangs Chisum's braces and walks towards the bandstand. Chisum is well away from the doorway at this point. However, in the next scene, Chisum is now standing near the doorway and Sonny is walking away from the bandstand.
Continuity mistake: Once during the dancing scene you can see Vince Fontaine standing on Marty's left side, and then suddenly on her right. (01:10:16)
Other mistake: In the opening animated sequence, it shows Rizzo taking off a top and her neck stretching. She looks in the mirror. As she is facing the mirror you can see that even with the elongated neck you still should be able to see her face in the mirror, but the elongated reflection just shows the neck looking much longer than it really should. Also, as the animation goes on you see Riz turning sideways but the reflection in the mirror does not match. She is sideways while her boobs in the reflection are facing the other direction. Then when she puts the black blouse on her back is to the mirror but we see her front in the mirror.
Continuity mistake: When the three T-Birds go to moon at the TV camera during the performance of "Blue Moon" at the National Dance Off, you can see there are two red balloons in front of Doody, but in the next shot the balloons have disappeared.
Continuity mistake: When Danny and Sandy enter the sports hall for the National Dance Off, there are four people behind them sitting on the spectator benches, but when Doody and Frenchy enter the hall in the next shot, the people have disappeared and there is now a woman in a pink dress behind the benches.
Continuity mistake: When Jan is mimicking the Ipana squirrel on TV, she put her hand up to her mouth when she is singing, 'Brush-a Brush-a Brush-a', and pretending to brush her teeth. In the second half of the song, however, after the second line; 'New Ipana toothpaste', she has both her arms pointing in front of her making a 45 degree angle with her body, and her neck straight but in the next shot, which is at the beginning of the next line, 'Brush-a Brush-a Brush-a', she has one hand up to her mouth again and her head is pointed a little to the right. You might also notice that the time between those two lines is longer than it should be.