I've experienced several instances of rude behavior most of which have been mentioned already, but this was a sad first -
I was at one of my favorite plays last night and during one of the garden scenes, there are several flowers placed on the front of the stage. Well, as this scene was going on, a woman in the front row actually reached out, put her hand on one of the flowers and laid it down on the stage (although the flowers aren't really big, I guess her view was "obstructed"). Now it was astounding enough that she had the nerve to touch part of the scenery, but then she did it again one minute later!! Of course the actors kept going completely unphased, though I was mortified. I think the woman and her party left during intermission or moved somewhere else, but I've never seen that happen before.
There were also quite a few people that got up in the middle of scenes as well as someone who came in late at the top of the second act, went to find his seat but when he couldn't figure out how to get back to it without stepping over people, just stood in the aisle for a couple minutes while an empty aisle seat was only a few rows away. And on top of all that a couple low volume cell phone rings during the second act... Ay me...
"All the world's a stage,
And all the men and women merely players:
They have their exits and their entrances;
And one man in his time plays many parts..."
~Jaques - "As You Like It" Act 2, scene 7