"And Steve perpetuates the myth that a roped rock climber hanging free has suffocated. This has happened in crevasses on glaciers, but I have never seen a documented accident report describing this phenomena on a rock climb. It can happen, but it doesn't. After having reviewed the literature, I have concluded that more climbers have suffered puncture wounds from pitons than have suffocated while hanging free on a rope on a rock climb. Fallen climbers have stood on prusik knots or employed the baboon hang to keep from suffocating. Consider this a challenge: show me an accident report about a climber suffocating while hanging from a waist tie-in on a rock climb."
Here is your report: "Accidents in North American Mountaineering", 1977, p.19-20, and also "Off Belay", December 1976.
Mitchell R. Haydon died while rappelling at Pinnacles National Monument. He was using a prusik backup to the rappel. The prusik caught, and he could not release it. The prusik was connected to a "waist tie of 6 loops of 1 inch tubular nylon webbing", and he was unable to release it. His breathing was constricted, and after about 25 minutes, he lapsed into unconsciousness, and started convulsive vomiting. 10 minutes later, he was lowered to the ground, but his airway was blocked and CPR efforts failed.
[WEBMASTER'S NOTE: This accident is also written up on RockClimbing.org, where the author notes "Tension must be eased within ten minutes or unconsciousness can result."]