I was at a very small show years ago in a church hall, and the notably emotional singer/guitarist whipped into a crouch, grabbing his head. People thought he was being theatrical at first, but he came off stage cursing (not into a mic), which was not common practice at a Christian rock show…
The mic had shocked him on the mouth as he stood in some spilled liquid. The band vamped for a while, not knowing what had happened, but he didn’t return, and that was the end of the concert.
I run sound at a NYC venue regularly, and fill in occasionally at a couple of others. I’ve had in the past three years maybe six or seven performers who’ve used a wireless mic – which they have supplied; I don’t know of any venue that uses them in a house setup. And other than for vocals (most of the “electrocution” stories involving amplified instruments, not vocalists), I’ve never seen any wireless setup… and wouldn’t want to.
That’s because bar owners are generally cheap bastards and an SM58 and a mic cord is a lot cheaper than even the cheapest wireless system. And touring bands are not to be trusted with expensive wireless systems either.
What I’m saying is house systems are a special case. And most singers who care about their health carry their own vocal mics these days.
I’ve worked in touring theater, which is all about wireless. Finding open frequencies is one of the biggest pains of the job, but the sooner the old analog systems are retired and replaced with frequency-agile digital ones, the better.