You can probably guess already that this has happened to me recently. Yesterday we were playing the acoustic tent at a huge local music festival and during our last two numbers a slightly scary lady first tried to grab my harp mic from me during my crowd walk, and then barged on stage. Ironically she complained to my wife afterwards, not realising who she was, that I'd been "very rude". My exact words to her were, "excuse me love, can I have my microphone back", which was very much the polite version of what I wanted to say (which I won't type out here for the sake of good taste). It's not the first time this has happened either, and I know from experience that letting them get to the end of the song means that they'll think they're being made welcome and be up during every number.
Apart from anything else I'd consider it bad manners for a professional musician to join me on stage uninvited, let alone an unknown drunk. Even in the relatively informal situation of an open mic, the very furthest I'd ever go is waving a harmonica at a performer mid-song whilst mouthing "would you mind?" from the wings. I have great friends who I've played with many many times over the years and I still wouldn't jump on at a gig without an explicit invitation to join them. I'm pretty sure they'd extend the same courtesy to me.
The flip side of this is that we don't really mind having people join us when they've asked nicely, we had a total stranger join us for a few numbers at a gig a year or two ago, just because he came up to us on the break, talked knowledgeably about music for a bit, and then mentioned if we were planning to play Crossroads he'd love to add some guitar.
But here's what I'd like to say to that lady, now when I've had time to cool off and ponder, and it isn't the expletive ridden phrases that I was biting my tongue to suppress yesterday. I work hard to be on that stage. I spend time and money seeking out gigs, I've given up weekends to gig, and Tuesday nights to practice. I run websites, I print posters, I read articles about marketing via social networks. I've been at it with some dedication for four or five years, even to be up there at a festival dedicated to local bands. I've gone from open mic nights to unpaid gigs to paid ones. I've played to one man and a dog at a country pub on a Sunday lunch time, and a rowdy coachload from Sunderland in a desperate Coventry estate pub on a Friday night. All of this has allowed us as a band to improve, to build chemistry, to reach the point where you and a tent full of people were enjoying themselves yesterday. I'm not professing to be a great musician, or even a terribly original one, but I've bloody well worked at it, and it's paid off in its small way - I've got better. How dare you presume to interfere with that, to have the arrogance to think that joining us on stage, without even the simple courtesy of asking permission, would somehow be anything but an annoyance to us, or the people who had come to watch us.