In the scene where they are singing Greased Lightning, when John is doing the chorus of the song--he says or his mouth forms the word heap lap trials instead of heat lap trials.
Well, maybe Travolta was just enunciating the word 'heat' too much. It probably isn't even a slip-up at all, the person that found it was most likely trying too hard. Either way, I do not find it to be that significant.
I think he just messed up what he said it and the director thought he was such a tiny mistake and since you can barely notice that he messed up that it wasn't worth doing the whole scene over again.
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