Several years ago an experienced party of Croydon cavers set off to do Marble Steps Pot in Yorkshire. All went extremely smoothly - all the pitches were efficiently rigged, all the gear was there, we even managed to find the cave first time! At the bottom of the 90ft pitch, the rest of the party went down the last little drop as I had been down several times before and didn't want to get a soaking. I decided to start prusiking out.
Reaching the main belay point at the top of the 90ft pitch, I reached up to clip the cowstails onto the rebelay line, as the top of the pitch ascends steeply upwards. As I stood on my foot loop to get a bit higher, there was a ripping feeling and my one and only light went out. I sat there gobsmacked - I had never in all my evenings in the pub talking about caving heard any one explain what you do when this happens. Do you sit there and wait? Do you let somebody come up behind you? Do you go back down? By now there were lights and shouts from below but it was too far and too wet to communicate.
I decided that the safest and quickest thing to do was to carry on by feeling my way, and to get off the rope and into the top of the pitch. Theoretically this should have been no problem as I was already cowstailed to a rebelay bolt, so I undid my waist jammer and tried to pull myself off the pitch head. This required tremendous thrutching but eventually I was securely wedged in where I would be able to use the light from the next person up to sort myself out.
I whistled down that the rope was clear but as the next person put his weight on the rope I realised that I was being dragged down - somewhere the rope was snagged around me and I had in fact pulled the rope up into the alcove. Feeling all the various pieces of equipment, I realised that the safety link between the foot jammer was the problem, and that if I undid this, it would release the tension. Getting the rope off the crab required tremendous effort but eventually I got it free. I then tried to let the rope out slowly so as not to drop whoever was coming up. Unfortunately the tension was too much and it slipped from my fingers dropping the person on the way up by at least 6 inches! This, as might be expected, caused considerable upset to the ascending person, who thought that either the rope or the belay was going. However, when he arrived I was easily able to move safely off the top of the pitch.
The moral of the story is that you should be able to do every SRT manoever in complete darkness with confidence (some people practice blindfolded).