In today's Washington Post David Segal writes about typical boorish behaviors he's encountered in his years of attending rock concerts. I can't say I've seen any of these first-hand, but it's been many years since I went to a club for a concert where people actually listened attentively to the music (as opposed to a P-Funk All-Stars show, which is about the dancing as much as the music).
The article reminded me of my bygone concert-going days, where we'd make fun of the people who came to the show wearing the t-shirt of the band that was performing. "That Guy is everywhere" was a popular concert quote. According to us (originally Brad, I think, though I'm sure someone will correct me if I'm wrong) dressing that way was exceedingly uncool. It was OK to buy a concert t-shirt at the show, but you couldn't wear it until at least the next day. Rob and I inadvertently created a corollary of this rule at a Who show in 1997 by wearing the same Van Halen t-shirt. Don't wear the exact same shirt as someone else in your group, especially if the band on your shirt has no connection to the one performing that night. Not only will your friends make fun of you, but so will people you don't even know.