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.
My friend and I saw Deep Impact in the theater, when the plan fails and the comet fragments are still going to hit earth, there is a dramatic moment when they explain to projected impact sites. One in the ocean and one ... in Canada. For whatever reason my friend and I both burst into laughter when they said Canada. The whole theater turned to glare at us.