I HATE anyone coming uninvited into the stage area. My worst one was back in my KISS tribute days at a gig in South Shields where a couple of drunken idiots got on a stage full of pyro and tried to sing along in the choruses. When they wouldn't leave, I stopped playing and left the stage, refusing to return until venue security did their job. We'd put up "Warning, pyro!" notices either side of the stage but you can just bet that if one of those drunks blew their leg off it would be OUR fault....
Shame, really..... would have served them right.... I love a bit of Darwinism....