Some years back I worked a residency, 5 nights a week for over 7 years. During the week it was just me on arranger keys and vocals and at weekends the woman who owned the place with her husband used to sing. She had been professional and had a pretty good voice even though she was no spring chicken.
Sometimes the owners friends would come around at closing time and they would pay me extra to stay into the early hours to continue playing but mainly to let their friends come up and sing.
They would use the mic the girl singer used which belonged to me which was a pretty good quality EV one.
One night, one of the "friends" got up to sing. I didn't do uptempo numbers because joe public cant keep the beat and I always try to work out their best key and give them a loud intro ending on the dominant 7th chord of the key to lead them in. If you make them sound good you get good tips.
The guy who was singing had sung before and had a half reasonable voice but this night he had been drinking and after I had played his intro he started to sing a completely different song in another key!!!
I have backed cabaret for years and pride myself on being able to follow the most dodgiest of acts as most of the dots they provide are rubbish and basic at best and I also play by ear.
As I was trying to find this guy's key he was trying to change it to find mine which ended up in a farce as you can imagine.
He stopped singing and glared at me. His friends, who normally give him a standing ovation for doing a reasonable job were now laughing at him which was more than he could bear.
He used the mic as a weapon in true Roger Daltry style swinging it around at me and smashing it against my equipment.
The mic was trashed but luckily enough the rest on my stuff and me were reasonable unscathed other than yet another scratch on the wood of my Leslie 145

The club replaced the mic but as they had purchased it they seem to think that they could get anyone up at any time after that to use it. I left soon after and I have never allowed anyone I don't know to sing and I never will again.
Do it at your own peril
Keith