For me it was Unbreakable, you want unintentional, there's a horrible bit where the guy with the brittle bones falls down the subway steps, and I'd been really into the movie up to that point...
Suddenly, I feel this tiny giggle escape, then I'm laughing with every snap of bone as he cascades down the stairs like a crunchy slinky, and I know the entire cinema is thinking
who's that asshole who's laughing at this, what a sicko' and that just makes everything funnier, knowing I'm being stared at disapprovingly.
Honestly I hate it when people talk loudly during movies or sit there making fun of a movie, but it was completely uncontrollable laughter and highly innaproppriate.