Knotting dyneema is much worse than knotting nylon slings due to the low friction coefficient of dyneema. So a thin sling could easily go from 22kN > 11kN.
In a sharp fall giving 4-5kN on climber, the force is doubled on the quickdraw. That's not a margin I'd be comfortable with. And of all climbing gear, slings are the most vulnerable to strength reduction from abrasion. Would you be happy falling on a nylon quickdraw half cut through? No, but only because it looks bad but a knot doesn't.
It's one of those situations which is unlikely to cause a problem unless other factors stack up with it, but of course, bad accidents when analysed almost always happen because several factors, none in themselves being critical, stack up to become so.
We know that getting a force of above 7kN on the top runner is very rare, simply because otherwise we'd get many more reports of karabiners failing than we do, as this is the minimum gate open strength allowed these days.It is possible though: a very hard catch, high fall factor, stiff old rope, poor bolting so there is lots of friction in the rope path or the rope snagging on something. More likely though, you get lazy. You set this up, fall on it and the knot is a proper fucker to undo, so you don't bother. You like the set up and carry on using it. The knot now is impossible to undo. I'll sort it out next time, one more redpoint before it gets dark though. Bam you're on the deck.You couldn't see the damage hidden by the knot and the knot being tight couldn't soak up any fall energy, plus the tightened radius on the knot increased the stress at the critical point. Lot's of small things stacking up to cause carnage.
If you are going to knot stuff, always undo them afterwards!
