Revealing mistake: In the soda shop, the waitress turns off the lights with her elbow because her hands are full, but she misses the light switch by 6 inches. (00:55:10)
Audio problem: At the cotton candy stand at the graduation carnival, Blanche is seen silently mouthing the question "How many?" In the next scene, from a different camera angle, the same question is asked - this time with audio intact.
Continuity mistake: When the three T-Birds go to moon at the TV camera at the National Dance Off, Doody says "1, 2, 3, go!" and pulls down his trousers to his ankles. In the next scene, his trousers are suddenly pulled right up to his buttocks.
Factual error: At the graduation carnival at the end of the movie, Danny enters wearing a letterman's sweater and explains to his buddies that he earned it by lettering in track. However, on his left sleeve are two stripes, indicating that he lettered in two separate years. Since you are only awarded a stripe for each year you letter, and since Danny was a senior when he lettered in track, his Rydell letterman's sweater should have had only one red stripe on his left arm.
Continuity mistake: In the scene where the whole gang is eating at the soda shop, everybody starts leaving. Doody is in the back, in the middle, on the railing. He's getting ready to leave, takes a bite of his ice cream and sets it down. In the next shot, he sets it down again.
Continuity mistake: In "We go together" they all dance forwards. When the camera changes angle Frenchy and Doody have switched sides.
Continuity mistake: Sandy walks over to meet the girls after doing a cartwheel. On Sandy's cheerleading uniform, there is a LONG red string right next to the letter on her shirt. Then, suddenly, the string disappears in the very next shot. Then in the scene after that it is back again, and then the string is back on her shirt. (00:22:20)
Continuity mistake: During the slumber party at Frenchie's, when Rizzo brings out the wine, check out the junk on the headboard. Later in the scene when Marty is putting on her robe, the headboard is clear and now has a black fan on the left. Then even later when Jan says "You ain't getting your hands on my ears" the headboard is cluttered again, with the black fan too. (00:27:35)
Continuity mistake: In the middle of the dance contest, Patty Simcox does a funny front walkover, and comes dancing towards the camera. Over her shoulder, you can see Sonny running up behind her. But suddenly it cuts to another angle, and Kenickie is running up behind her to lift up her dress, and Sonny is nowhere to be seen.