My worst experience was in Lord of the Rings: Return of the King when I sat in front of an Asian family. Only one of them had seen the previous two, so explained to the others every intricate plot point that had previously occurred. The young children sat behind me, kicked my seat, one got sick of sitting so decided to stand, leaning incredibly forward so I could feel her breathing on my head. With the scene at the end when Frodo kisses Sam, one kid turned to the other and said "Are they lesbians?" and the other went "No. That one's married".
The only thing comparable I've experienced was in Titanic when, in the scene wherein they try to avoid the iceberg, someone shouted out "OH MY GOD!!!!! IT'S GONNA SINK!!!!"