I was preparing to ab down to Fisherman's Point. When I weighted the rope, it popped out of the device - I had failed to clip it. I had firm hold of the rope with both hands and was able to pull myself back into balance and sort it out. I then descended without incident. I was shocked because I am very wary of abseiling and usually check everything very carefully. Despite this, I somehow failed to spot this error. I have been trying to work out how this happened. It was the fourth day of the trip so perhaps abbing had become a bit routine, and it wasn't an intimidating ab (although the consequences could have been very serious). I think that despite my paranoia about abbing I had perhaps become complacent, and although I checked my setup maybe I didn't do this in a properly structured way. Perhaps I was already thinking ahead about how best to get over the edge. Also, I use a self-locking krab with a retainer to prevent cross-loading - this had become twisted in the sling and I had to sort this out, which perhaps distracted me from checking the whole set-up properly. Being self-locking also meant I didn't need to check the krab again, but if I had I might have noticed the rope wasn't clipped.