In addition to the screaming and singing loudly that everyone is talking about, I also hate when people constantly whistle/hoot/holler/etc. or are booing any acts that are on. I understand you may not like everyone performing, but booing is not necessary no matter what. And seriously...if you're booing the headliner then you have no business even still being at the show since the artist(s) you came to see are over.

2 years ago I went to see Jack's Mannequin and Paramore (Paramore being the headliner) and this jerk behind me kept booing Paramore SO loud and screaming nasty things while they were on. I had to turn around and scream at him and he finally shut up. I don't understand why he didn't just leave after Jack's Mannequin was over.