This incident highlights many of the reasons accidents happen. The reason Fred chose to use this knot was that he had seen me tie one earlier in the day at the same abseil station and thought it seemed better than the method he had used in the past (a reef knot with a barrel knot either side - i.e. a reef inside a double fisherman's). In fact, I had tied an overhand with 45cm tails. Fred misidentified the knot, didn't now that long tails are needed for any such knot, so used shorter tails, didn't know that figure 8's can roll and continue to roll at low forces and that they need precision dressing, and that they have been implicated in previous accidents- i.e. are best avoid. I knew all these things. Fred decided to use a technique new to him for the first time in a real situation where failure might be fatal. This seems a common issue in climbing, where people use a new device or method without reading the instructions or seeking instruction. I made the following errors. I had never climbed with Fred yet failed to ascertain his level of experience, taking a willingness to lead an HVS as proof of enough experience to cover common things like abseiling. I didn't check the knot, which given I was climbing with someone I didn't know the competence of was stupid. And this is where it gets a little bizarre, I did clock the knot in passing. However what I saw, was a Flemish bend (i.e. a rethreaded fig 8, where the tails exit the knot from different ends of the knot) with short tails. This is a very strong knot and was a common way of joining abseil ropes in the UK before people believed in the overhand. I used it a lot once upon a time. This needs very little in the way of tails. In my head, I can still see this knot at the station, with the tails exiting different ends of the knot. However, all I saw was the first thing that came into my mind that was somewhat similar to something I had used, I didn't check the knot in any true sense. This is identical to yanking a grigri to check it locks, without making 100% sure you are yanking the end really tied to the climber. The take away message - if you are doing something for the first time, don't do it live, read the instructions and ask someone more experienced to triple check; if you are climbing with someone you don't know, it isn't sensible or fair to assume they have everything dialled.