In reply to David Smith:
I once held a mate who took a good 20ft fall onto a Screw on the RH nameless stream near Idwal, and he's still around and smiling. I was taking in the slack he was creating as he came past the screw
I have a mate who held a guy who took an 80ft whipper off weeping wall in Jasper a few years ago. There was a lot of rope slippage in the process (iced ropes, gloves, climber with backpack, etc). The leader ended up below my mate on the belay, luckily it was so steep that he didn't hit anything on the way down. The screws held and they lived to climb another day.
I heard that it used to be the case with a lot Ti screws that came out of Russia, that the hanger would often fail before the ice.