In reply to Duncan Bourne:
Came very close to sending a Novice Friend of mine plummeting to almost certain death.
Was climbing in on Milestone Butress, Northwales in a 3 with me as the Leader, an Experience Second and one Novice. I geared up and set off up the first pitch, trusting my belayer to sort out the newbie (helping him harness-up and tie in), sending him up after me with my belayer coming up last.
We top out after 3 pitches and I set up a ~50m ab to return to the ground. My belayer goes first to sort the ropes out as he descends and to show the newbie how it is done and of course I go down last.
Set up on the ab, the newbie backs towards the edge of the gulley and then begins to weight his harness. As he does, the waist belt of his harness suddenly pops!!
Fortunately he has a firm grip on the rope and he remains upright and doesn't tumble out of his harness completely. I drag my very shaken friend back to the safety of the cliff top.
It took quite a while to work out what happened, the harness looked to be in perfectly good condition and there was no reason it should have slipped if put on correctly.
I started to question how my friend had put on the harness, 'Did you put it on correctly?', 'Yes', 'Did you double back the buckle?' - 'Yes', it seemed like he'd done everything correctly.
He had no wish to put his life in the hands of a bit of gear that had failed him once, and there was no other way off the mountain that I knew of. I kept insisting that if it was put on correctly there was no way it should have failed and if I supervised him putting it on it should be fine. I even offered to swap harnesses with him so I had the 'dodgy' one. But he was not happy (understandably).
Eventually I got him to show me how he had put on the harness, it turned out that when my belayer had told him to double back the buckle, he had re-threaded the waist belt back the exact same way that it came through on the first pass, so the only think holding the harness together was the friction of two layers of webbing passing through the buckle instead of one.
Once he understood the mistake he was prepared to trust the equipment again, when used correctly and we managed to get him down.