So last week, first night of a 2-night engagement at a bar we'd never played before. We get done with our first set, things are going pretty well. All the sudden these 4 dudes walk up to the stage with their guitar cases and before I know it the bassist is plugging into my amp and the drummer sitting down at our kit. I ask my BL what the heck is going on and he says the owner wants us to let this band play (on our gear, through our P.A.), while we're on our break. My drummer and I are mad as hell and the BL says he had no idea this was coming but says we just have to go with it. So they play 4 or 5 songs, thank us for letting them use our gear, pack up their stuff and leave. Meanwhile all these people in the crowd are coming up to the guys in my band and wondering what's happening and saying this other band sucks and they did it to the band who was there the night before us as well. Then when we get back up for our 2nd set I had to completely re-do my amp settings cause the other bassist had screwed them up so bad even though I asked him THREE TIMES to use a different pre-set on my amp and leave my settings alone. If this had been a big-name band who just happened to drop in I could totally see it, but these were guys nobody had ever heard of before. So it left us all just scratching our heads. I mean really... I've been playing live music for almost 30 years and this has NEVER happened to me on a gig before. Anyone have something similar happen or know what might have been behind it?