I was out climbing with friends one day and I let a stranger (who looked an experienced climber and had been belaying other people too) belay me. I yelled "tight" and I saw her actually letting go of her brake hand when anyhow taking the rope in without proper procedure (she was just using an ATC).
After that incident I told her of that possibly fatal mistake and she just shrugged it off!
no such problem. lead climber all the time.