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)
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.
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.
Revealing mistake: When Sandy is at the shop with the jock, then she goes to the jukebox, the picture behind the jock is all blurred out. You can tell it is a Coke sign - Coca Cola cancelled their sponsorship after filming. (00:43:30)
Answer: It is implied that Danny and Rizzo had a previous romantic relationship of some kind. When Jan says, "Did you guys get a look at Zuk this Morning? Looking pretty good this year, huh, Rizz?" Rizzo replies, "that's ancient history." Later, when Danny says, "you're looking good, Rizz, " her answer, "eat your heart out," also suggests some possible residual feelings on her part. I think that's why Rizzo enjoys tormenting Sandy and Danny by reintroducing them at Rydell at a moment that she knows Danny is going to save face around his friends.
Michael Albert