I once worked alongside a musician who just turned up with a mixer and his Keyboard, Guitar, Mic...but no PA.
I knew what was coming and as soon as he set up he said "Here are mate, plug this in for us."
I said "Why can't you plug it in your own PA system? I don't let anyone plug in to my system unless I know them and have agreed it in advance" and he was like "Well, I assumed it would be ok, everyone else lets me."
You could tell it had wound him up, but his attitude from the moment he walked through the venue door just screamed '
hole'
He tried to kick up a fuss, to which I said "Look, i'm the one with the PA system and you're the one who's going to be up the creek without a paddle once the bride and groom find out, so I suggest you get off your high horse!"
In the end I let him plug in to my PA, but I told him that I will control the levels and if it's not as loud as he usually has it 'tough', he should've brought his own PA system!