I get that kind of stuff doing live sound in this venue too, its small enough and the Foh really is only re-enforcement of whats going out on stage- mainly when i do sound there its vox, guitars, bass and kick-sometimes toms but usually not. Drums are crazy loud due to place the stage is at.
Sometimes bands crank their amps between soundcheck and going on-( hate that, defeats the point of the sound check!) and vocals can't get louder cause of feedback but are still low in the mix because of the amps being so damn loud. There's nothing you can do till they finish the first song and tell them to turn them down again, but people come up to you and say "I can't hear the vocals?", no shit- no one can! Also happens sometimes when a band doesn't get a soundcheck, you do a quick level check, then they start and you see the problem.
Doesn't happen often but when it does, the punters are dicks.
Also I find both live and recording that those with a little bit of knowledge are the most annoying, you feel like getting up, and handing control over to them for 5 minutes, see how much better they have it when you come back, ya know.
I know one guy who's never done live sound or been at the helm of a proper recording but cause he knows a little about sound in theory drops remarks all over the place- had 1 band who's amp had a spot of trouble before a set so the guitarist fucked with his settings, they played their set but his tone was a bit crap, I was only running him through the p.a a tiny bit and giving him boosts for solo's. My friend kept saying to everyone really loudly while he was around me " pity about the guitar sound, ya know" and smirking over at me as though he would have fixed it. Even the guitarist in the band- who's also a sound engineer! -admitted to me right after their set that it was his fault with the tone and there was nothing I could have done to remedy it. The guy wouldn't leave it go and to this day when the band gets mentioned he drops that comment in there, pisses me off to no end!