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!
Sandy's hair, throughout the whole movie, is different lengths. You can notice especially when she has a ponytail, as a cheerleader, in the pep rally - her ponytail is longer than her hair when it's down.
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 clearly see she is wearing knee pads.
During "we go together" at the end of Grease, John Travolta has some problems with mouthing the lyrics.
In "We go together" they all dance forwards. When the camera changes angle Frenchy and Doody have switched sides.
All of the characters (except Sandy) speak with various kinds of New York accents, some of them very harsh, even though the movie is set in Los Angeles. You'll find New Yorkers anywhere, but that many in one place 3,000 miles away from home?
In the soda shop scene where Sandy and Danny are at the jukebox, the reflection of the boom mike is clearly visible in the chrome part of the jukebox.
In the end during "We Go Together", there is a scene where Sandy jumps towards Danny and she doesn't have her red Candies shoes on. [She's holding them in her hands].
Sandy walks over to meet the girls after doing a cartwheel that sucked. They are telling her she did great. Anyway, if you look at Sandy's cheerleading uniform, she has a LONG red string right next to the letter on her shirt. Then, suddenly, the string disappears in the very next second.
When "Crater Face" comes to the dance with Marty and Rizzo, he takes his sunglasses off twice.
At the beginning of the movie in the hall everybody closes their lockers even though the locks are locked.
At the beginning you see the coach scouting basketballers when track is going on. That same day the coach is getting the T-Birds ready for football. It's like the school has all three major sports going on at the same time.
Why is there a big hole in the middle of the football field?
How can everybody hear the T-Birds & Pink Ladies during the song at the bowling alley.
When everybody was dancing on the bowling alleys, wouldn't they be falling down because of how slick the lanes are?
When the twins are singing about Brad for the talent show, and Michael was playing the piano (you have to listen and watch), his hands are suppose to go across the keys but he is still doing the fake up-and-down-hand-move.
When Damucchi and Sharon go into the fall-out shelter, Sharon is carrying a lit candle. She blows it out before they both enter and then in the next shot the candle is still lit and she blows it out for a second time.