I actually work in a theatre, so I have a few stories that are from that perspective rather than as an audience member.
Opening day of The Dark Knight we had some pretty long lines, as I'm sure you can imagine. I was in the hallway watching the line to get into the theatre (the previous show was still playing). The line was already fairly long, but not so long that if someone got in at that moment they would have to get a front row seat or anything. So, while walking the line, I noticed a family of four to five people siting on a bench across the hall from the line. I thought they were probably waiting for someone to get out of the restroom or something like that, but when they hadn't moved after about five minutes I decided to approach them and ask if they were here for the next show of TDK. They said yes, so I suggested that they needed to go ahead and get in line. One of the guys said they'd been there for "a long time" (they hadn't) and figured they shouldn't have to wait in line; they should get in first.
I apologized and told him that no, he wasn't going to go in first. I told if he hopped in line now he could still get a pretty good seat, but if he stayed on the bench I was going to have to let in the line completely before I could let him in. He replied, "You know what? You're a moron. I want a refund." I said, okay, and took him over to guest services. When he got there he told the GS manager, "I want a refund because that guy there" pointing to me "is a moron." While he was getting his refund his little kid was asking, "Daddy, why aren't we going to see the movie?" He replied, "Because that guy's a moron." After he finally got a refund and was about to leave I said, "Good night," and he replied, "Moron."
I've also caught being doing things... they shouldn't be doing in the theatre.
A few years ago I was doing an auditorium check (making sure the picture and sound and audience are all good, etc.) on Reno 911. We have to walk through the entire auditorium, so I first walked down to the screen, turned around, and saw that there was no one in the auditorium. I was about to leave (no point if no one is there) when I saw a flash of white all the way up in the top row. I immediately thought it could be a couple enjoying each other's company too much, but I thought, "No way."
I started to walk up the stairs. When I got up to the top row there was a couple on the floor, completely naked, going at it like rabbits. I was dumbstruck; this was the first time I'd caught someone doing it in my theatre. I guess my keys jingled or something since they heard me and stopped. I couldn't say anything, so I just pointed to the door and shook my head. They started to get dressed when the guy asked, "Can we get some privacy?" Several "witty" comebacks came into mind ("Its a public place," "See that camera pointing right at you") indicating how little privacy the movie theatre had, but I just turned around. They left after that.
On another occasion, a couple of years ago, an auditorium was about to let out, so my ushers were getting set up to clean it. One of my team members, an old lady (late 40s maybe), was going into the theatre to check on capacity and see how long the movie had. In the doorway, right behind the door, was a couple going at it vertically. When the door opened the guy pulled out and turned to the usher, just hanging there. She noticed that, although they were stupid enough to do it in the doorway of a theatre that was not at all empty, they were responsible enough to use protection. The condom fell off as the guy rushed to bathroom and the girl followed to wait right outside.
The team member told me about this experience (it apparently really affect her; she indicated that "she hadn't seen a man since..." before I shouted, "TMI!"), and also pointed out the girl who was still waiting outside the men's restroom. I walked up to her, asked if her boyfriend was in the restroom, and said we'll wait for him to come out.
When he finally came out I pulled him over and told him to pick up his nasty condom off the floor. He denied it was his very firmly, but had no issues with picking it up and throwing it away. I told them next time I would call the police and escorted them out.
Its fun times.