Other mistake: On the first day of school, when Patty is starting to sit down next to Sandy, her reaction to sitting on the apple is way too early.
Other mistake: This mistake occurs during the dance sequence 'You're the One that I Want.' John Travolta and Olivia Newton-John have just emerged from a 'Shake Shack' and are now ascending a ramp. For the lyrics ,Hoo hoo hoo... honey,' the camera zooms in close to their faces. You can see that the dance move is for the two actors to bob their heads close to each other's so that they nearly touch. They do it right the first time, but the second time they visibly mess up. John Travolta seems to try to fix his mistake, but Olivia Newton-John appears to crack up and leave the frame.
Other mistake: The entire "Greased Lightning" number repeatedly goes in and out of focus.
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.
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.
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).