In reply to Offwidth:
We had a mini-epic coming off the Idwal Slabs a few years back. We were in full numpty mode that day. The weather was iffy, so we wandered around in Llanberis shopping, waiting for it to clear up. It didn't, so late in the afternoon we decided 'damn those torpedoes' and we would do a route in the rain in full wet weather gear with big boots on.
We started our climb at about 4pm, and the route went well but got a bit trickier higher up. We may have been off-route a bit (guidebook descriptions and pictures were not as good as these days - obviously it couldn't have been our fault)! For some reason we were climbing in series in a connected group of 4 (I think most of the group were not very confident at leading in the pouring rain in big boots) and we ended up with rope spaghetti to untangle at the belays on more than one occasion. Despite our multiple attempts at ineptitude, we made it to the top of the route before it was fully dark, and scrambled upwards in search of the way down.
We knew the descent gulley was located upwards and off to the left. My headtorch was usefully tucked away in the car at the bottom. After ascending for a while we luckily found an arrow scratched on the rock which pointed left then downwards. We traversed left for a while then descended a small gulley in the dark, which we figured was on the descent route. The down-climbing felt quite intimidating but got easier. All went well until we found ourselves back on the same ledge that we had topped out on at the end of the climb!
We decided it was a bit late to be trying to find our way off the route in pitch darkness, and made the decision to stay on the ledge for the night. We spent the night huddling to keep warm. Luckily the rain had stopped. I was trying not to doze off so my contact lenses would not stick to my eyes - I didn't have a spare pair and would not have been able to see anything in the morning if I fell asleep. Also the ledge was sloped, so dozing off could have been a bit precarious! I remember being really tired and lapsing into waking dreams a couple of times.
Anyway, all ended well and finding our way off was much easier the next morning with the benefit of daylight.
The day would have been utterly unmemorable if there had been clear arrows or cairns showing us the way down!