I made the mistake of going to a late (12:40am) show of Project X (terrible movie) with a friend this past Saturday, and it was without a doubt the worst theatre experience of my life.
As you would expect with a movie like Project X, the theatre was filled with a loud and unsavory crowd of people. They treated the theatre like an outdoor park and chatted the entire time. When the film began and some loud song I didn't know started playing, people not only started singing along but they got up out of their seats and started dancing. As the film's mayhem progress it was as if a party erupted in our theatre, with people dancing, chatting, yelling, etc.
Worst of all, a few people actually took out joints and started smoking weed. In a public movie theatre. Luckily they were escorted out but it caused a scene and it caused the theatre to smell.
I couldn't even believe it.