People who block the screen, especially in a movie with subtitles. Seriously, that has got to be sin number one for the movies.
I went to go see Inglorious Bastards with my dad in a very VERY small movie theatre. We had great seats, which were couches [sub](yes, COUCHES)[/sub]. It was great. But there was in old lady up front, who had a hat on, blocking the bottom half of the screen. It wouldn't have been TOO much of a problem, but since half the movie was in German and French subtitles, I couldn't understand a damn thing they were saying. I had to rely on context. And I couldn't have said "Down in front" or something because:
A) She was an old lady. You just don't do that to an old lady.
B) She was kind of far away, so you had to raise your voice to get to her, which would disturb the other members of the audience.