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.
Audio problem: During "We Go Together" at the end, John Travolta's mouth is not in sync with the words "we'll always be like one." He appears to start singing "that's the way it should be" before realizing and correcting his mistake.
Continuity mistake: At Frenchie's sleepover when they are talking about piercing Sandy's ears, it shows Sandy holding the wine bottle and passing it to Jan. Then it cuts back to Sandy and she is still holding the wine, then it cuts back to Jan and she has the wine again.
Revealing mistake: At the dance, there's a scene where everyone started dancing after Cha-Cha and Danny won the trophy. Well Marty comes in from the side of the screen and lands on the floor in some kind of split, and you can see she is wearing a knee pad on her right knee.
Continuity mistake: When "Crater Face" comes to the dance with Marty and Rizzo, he takes his sunglasses off twice.
Continuity mistake: In "We go together" they all dance forwards. When the camera changes angle Frenchy and Doody have switched sides.
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 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.
Visible crew/equipment: In the soda shop scene where Sandy and Danny are at the jukebox, the reflection of the boom mic is clearly visible in the chrome part of the jukebox. [True of videos and early DVD releases - in later DVD releases the reflection has been removed].
Factual error: When the Pink Ladies are walking Sandy into the outdoor cafeteria, just after she tells them the name of the boy she met last summer, all the cars in the background are from the 70's, not from the late 50's or early 60's. This actually happens twice in the same scene.
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.
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: 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.
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.
Continuity mistake: There is a scene in which Danny falls over a hurdle. If you watch his falling frame by frame you will notice that only his right leg is stretched first. Then the camera angle changes and suddenly his left leg is stretched, while the right one is bent.
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.
Revealing mistake: When Sandy checks into school for the first time, she goes down to the office and the assistant principal gives her some forms to fill out. If you look closely, you'll be able to see that the top form is just a blank piece of paper.