I've had similar to Keith's story happen to me. I was a drummer that would with a click and backing tracks, so I wore headphones. The singer was across the room facing me, with two EV 15" floor monitors from the 80's (The ones Bruce Dickenson always had his foot on) being feed by a 500 watt crown amp.
I could clearly hear the vocals through my headphones with the click track, bass, keyboards and guitar blasting and over my drums. As I said, she was 20 feet away and the wedges were pointing away from me. Singer couldn't hear herself. She never figured out that all that sound that was keeping her from hearing herself was her voice. (her voice was the only thing in the wedges)