A recent and relative experience: We were doing Englestead this summer on a 300' Imlay 8mm with only one canyon worth of use on it. We lowered the first guy (me, everyone else wanted the real deal) and had two rap down without any troubles. Though it was their second canyon ever they were both climbers and both rigged plenty of friction.
The fourth guy down used a Pirana and had on leather gloves. About halfway down he went to add friction around the lower and upper horn at the same time and missed. He was unable to recover and lost control. I was belaying and not paying as much attention as I should have. He (pretty calmly) yelled down for a belay. I gave a first tug and it didn't seem to do anything so I really pulled hard (I also had on gloves), nearly falling to the ground with the stretch in the rope. It probably took 40 ft from the time he requested a belay until I stopped him, and add another 10-20 for an inattentive belay. Shook us a bit, and surprised me at how long it took me to stop him...I had stopped him on other rappels several times before just messing around and it was almost instantaneous.
A few lessons learned, your experience may vary:
- It's a good idea to switch up the belayer, especially on a long rap with a large group. I was less attentive because my neck was strained from looking up for so long.
- Make sure you know how to use your rappel device in the situation you face (in this case a long rap with a skinny rope). Though slightly awkward, it would have been safer if he had added the lower horn earlier and then added the upper horn.
- If you are going to provide a firemans belay (and why wouldn't you?) do it right.