There are no statistics on how many climbers set off up a climb, either indoors or out, and forget to tie their knot properly in to their harness.
Lynn Hill is perhaps the most famous climber to do this when climbing at the Styx Wall at Buoux, France in 1989. On clipping her rope in to the anchors at the end of a route, she lent back to lower, the rope pulled out of her harness and she fell 70ft. She crashed through a tree and landed between two boulders. She was lucky, she suffered only a broken ankle, dislocated arm and a host of bumps, cuts, and bruises.
Other climbers have not been so lucky. Several climbers have died when they have forgotten to tie their knot properly, many others have sustained serious injuries.
Ask any experienced climber and they will no doubt tell you of many near misses, perhaps some of their own.
Personally I can remember a climber at Malham Cove (35ft fall, two broken ankles), one at the Owens River Gorge (65ft fall, multiple breaks) - one fell off then hit the ground, the other clipped the anchors sat back and then hit the ground. This year I saw a memorial to a climber who died at a crag in France, after they forgot to tie in properly and fell off.
In the last few months we have heard of five climbers in the UK who have forgotten to tie their knot properly in to their harness.
ALWAYS CHECK YOUR KNOT BEFORE YOU CLIMB AND CHECK YOUR PARTNERS KNOT BEFORE THEY START TO CLIMB.