I remember in Revenge of the Sith, when the actor playing Vader yells "nooooooooo", at what is supposed to be the emotional climax of the trilogy, the theater roared in laughter.
In Before the Midnight, at the theater with my wife at the time, when the couple are at the hotel having a serious argument. I was losing it, laughing uncontrollably, because it mirrored my abusive relationship I was having with her so damn perfectly.