don't be "that guy". there is a good reason why you shouldn't be "that guy". first, you are already showing your support for the band by attending their concert and cheering when they play songs. second, by wearing the tshirt of the band you are seeing, you give the impression of being one of "those people" who go to a concert to see one band and one band only. "fuck those opening bands. fuck the headlining band. i came late and am leaving early so i only see one band, my band, the band whose tshirt i am wearing". i have seen it happen. a bunch of kiddies came to iced earth/in flames with their inflames shirts on, and left before iced earth took the stage. they were "that guy".