Thing that always surprises me is when an artist starts ranting at the sound-man during sound-check.
I've acted as support to one-time "names", often in the twilight of their careers, who, presumably out of frustration at their reduced status, vent themselves at those around them.
I always likened it to insulting the waiter before you've had your meal. Not advisable.
On one particular occassion the sound engineer was having such a bad time at the hands of the head-liner, who wouldn't allow me to soundcheck until he was satisfied, (which wasn't goping to happen in this lifetime), that he pulled me aside and told me not to worry. "I guarantee you'll sound great" and "Fuck him", gesturing to the stage.
I thoroughly enjoyed the on-stage experience of quality monitoring and being in the hands of the sound guys. It's impossible to tell what's being projected when you're on stage, so you have to trust those at the desk and let them do their work.