I wanna play! OMG, there are so many. OK, I think it was the ANSUN tour, When they were doing the 70s/Jackson 5 part, JC dropped his (handheld) mic and he and Justin cracked up! The other guys didn't notice and just went on with the song.

In that same show, Justin, Joey and Lance were in the background doing dance moves, and there was one move where they were all supposed to jump and turn around. Well, Justin turned the wrong way, ended up facing Joey, and they both cracked up again! That was a great show, lol.