Continuity mistake: During the sleepover, Rizzo gets off of the bed with a cigarette in one hand and the wine bottle in the other. In the next shot, the cigarette and the bottle have switched places.
Other mistake: The entire "Greased Lightning" number repeatedly goes in and out of focus.
Continuity mistake: After Vince Fontaine begins the National Dance Off with Johnny Casino & The Gamblers, Mr. Rudie is seen walking down some steps near the stage, but in the very next shot he is seen at the bottom of the steps, putting his hands up to Vince.
Continuity mistake: In the song "There Are Worse Things I Could Do," between the lines "Out of spite or jealousy" and "I don't steal and I don't lie", Rizzo's hand jumps from the side of her book to the top of her book, near her face.
Continuity mistake: During the dance, there's one song where the guys stand in one line and the girls in another, and the couples take turns walking down the middle. In one shot, Danny is placing Sandy in line, and a couple (the woman has a blue dress on) is already starting down the middle. Then it cuts to another angle and no one is in the middle. Back to the first angle, and people are in the middle again.
Audio problem: When Sonny says "Shut up!" to Putzie outside Kenickie's car, his mouth doesn't move.
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: In the scene where the whole gang meets at the diner, the waitress brings out Danny and Sandy's food, and you can see the drinks they have ordered are a dark pink colour. In the next shot that they appear, the drinks are a lot lighter shade, almost pale pink.
Continuity mistake: When Marty and Rizzo walk past the TV truck, Marty says "Rizz!" and they stop and look at Kenickie, whose head is tilted to the right, but in the next scene his head has moved to the left.
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.
Visible crew/equipment: When Sonny snaps Tom's suspenders as he enters the gym, you can see a microphone in the top right corner.
Other mistake: At the dance, students are still arriving as the band starts to set up. A moment later, the MC announces over the mic that Johnny Casino and the Gamblers will get things warmed up, and then music starts, which is not possible with the band just setting up.
Continuity mistake: When Sandy is asking Rizzo if she's going to Thunder Road, her hair is curled under. When she's watching the race and then talking to Frenchy after Danny wins, her hair is flipped outwards.
Continuity mistake: During the basketball lesson with the team and Danny, Coach Calhoun is holding the ball and tells him, "You have to dribble." At this point you can see that Danny is very close to the coach, but in the next shot he is further away from him.
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: 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: Widescreen version: When Vince Fontaine introduces Johnny Casino & The Gamblers at the National Bandstand, Blanche is seen walking off the stage. In the next scene, she is on the stage, dancing with the band.
Continuity mistake: At the dance when they do that partner dance down the middle of the gym, as the lines form Kenickie and Cha-Cha start dancing down it, then the next shot its another couple first down it, and then a couple of shots later Kenickie and Cha-Cha start going down the lines again.
Continuity mistake: When Danny is running on the athletics track, he falls over one of the hurdles. He then picks him up and Sandy goes over to see if he is okay. You can see that Danny is walking towards to the field section of the track, but in the next shot he is walking away from Sandy.
Answer: Watched that scene again now and I just think it was poor writing (in what's still a classic film regardless). How none of the judges noticed that Cha Cha took another girl's (Sandy's) place even after she'd already been "tapped out" is another huge plot hole. Plus a lot of people in America were watching the dance on TV and it's not mentioned by anyone after this scene. 35 years later and that scene still bugs me (haha).