People who talk during concerts. Not music-appropriate "woo's!" or "Yeah, Steve!" or "Look at that ass!" (to me). I'm talking conversations.
I went and saw Steve Hackett last night. 500 seat theater and I'm third row center. Couldn't be better. Except... this woman right behind me kept saying "So good!" during the songs - I don't mean once or twice at the end of each one, I mean almost like what you'd think was "Tourettes"*. EASILY 75 times or more. Okay, I can live with that, I guess. But the set was an hour of a celebration for Spectral Mornings, and a hour and 15 or so of Genesis (he played the Selling England album in it's entirety, then ended with a supposed unrecorded song from Pete, Dance On A Volcano and Los Endos). But at the break her and her friend refueled with wine and the other one basically just talked the entire second half of SEBTP. That wonderful quiet part in Cinema Show as they build up to the Tony solo (played as a keyboard and clarinet duet; AWESOME, by the way)? "blah blah blah... So Good!... blah blah blah... So good!... " Add to that two flash photos and drumming/tapping that was out of time to the song (Battle of Epping Forest is admittedly not the easiest song to tap along to but still).
Didn't ruin the show or anything - it was very good, better than I hoped, not as good as it could have been - but certainly "irritating". The guys next to me, when the show was over, turned to me and said "Wow! What'd you think? That was SO GOOD." and sort of turned to the women, but they had already bolted while the band was on stage taking bows.