At the ending of the movie, they are all singing, "We Go Together".
At the point where a few of them have their arms around each other,
they are singing and watch, Danny (John Travolta) messes up in his
singing. You can't hear it, you see it. His mouth doesn't match the words.
After he messes up, Sandy looks at him and they laugh. You may have to go
in slow motion, look carefully at just those two people.