Realistically, there are still several single points of failure in the system:
1) The space between the belayer's ears
2) the rope if it has become damaged since it was flaked and inspected or gets cut while catching a fall. Yes, that happened, though not to me .
3) The protection on the wall, due to improper use. although depending on how high you are, the next piece down might still keep you off the deck.
4) The harness itself, although barring invisible damage this should be caught by the belayer/climber him/herself and the cross check.
5) The protection on the wall due to gear failure.
The above are listed in what I would consider descending probability, although 2 and 3 are pretty close, with 3 probably jumping 2 in trad climbing. I would rate the probability of 5 being negligible barring manufacturing defects, the prevention and possibly detection of which is probably outside the expertise of virtually all climbers.
Moral of the story: safety is complicated. (edited to add) I'd believe that trying to get people to apply a simple model of "how many mistakes" is better than none, but I wonder to what extent it reduces safety by making people complacent about having done a risk analysis when the system is too complicated to be analyzed that way.
Now I just wish that some of the people I see climbing outside (and inside!) would think as much as you're asking the 10-year-olds to. If they did that much they'd be much safer, nevermind a fuller analysis of the system.
I should also note that I am in no way professionally qualified to talk about safety in climbing, or risk analysis in general. I just like to climb, and prefer to do so safely.
(Part of the problem in this case was not understanding how the mechanism worked, which is not a good idea when it's the mechanism that stops you from falling 30 feet onto the ground.)
Gear failures of all kinds are extremely rare.
Edit: objective hazard is significant, too: rockfall, afternoon thunderstorms, etc.
1. Belaying off the end of your rope (Always tie a safety knot!)
2. Not wearing a helmet.
So, yes: climber error.
While climbing at the Gunks in NY about 8 years ago on a spring day, we heard this massive thunder a few hundred yards away. A small volkswagon sized boulder had broken free from the wall and fell smashing trees below. My friends were about 3 routes away from it when it fell.
There's good and bad news on the figure 8 itself. The good news is that they're visually easy to check -- any screw up will make the knot look very different. The bad news is that they invert at relatively low loads. That's mitigated with a long tail.
However, I've never seen any evidence that those single fisherman's people tie in the tail actually do anything. The key is making sure the tail is long enough that the knot is still tied if it inverts.
That's crazy about the rockfall -- lucky you guys weren't standing underneath!
I oversimplified my knots explanation so i wouldn't bore anyone. Isn't the fisherman's knot supposed to jam up the knot if you missed a loop of the figure 8?
There are also errors in judgment, but the gear is the center of this story.
The final paragraphs are still hard for me to read, six years on.
edit: adding National Park Service summaries
NPS Morning Report for October 26, 2006
Yosemite National Park (CA)
Noted Climber Falls To His Death
Todd Skinner, 47, of Lander, Wyoming, was climbing a route on the Leaning Tower when he fell approximately 500 feet to his death. Skinner's climbing partner reported the fall at around 4 p.m.
Skinner was pronounced dead at the scene. He is survived by his wife and three children. The exact cause of death is under investigation.
[Submitted by Adrienne Freeman, Public Affairs]
NPS Morning Report for October 30, 2006
Yosemite National Park (CA)
Follow-up On Fatal Climbing Fall
Jim Hewitt reported that he and his partner, well-known climber Todd Skinner, had been working on a first free ascent of the "Jesus Built My Hotrod" route on the overhanging west face of the Leaning Tower. Skinner's fall occurred when he was rappelling.
Hewitt told investigators that he had been above Skinner when he fell. As he was rappelling on the low-stretch ropes that they had fixed on the route, Hewitt came to Skinner's Grigri descent device on the rope at the point where he had fallen.
The Grigri had a still-locked carabiner attached which had been connected to Skinner's harness. When Skinner's body was recovered, the belay loop on his harness was missing.
The next day, rangers recovered a broken harness belay loop in vegetation at the base of the wall. It was very worn at the spot where the break had occurred.
Hewitt later told investigators that Skinner was aware that the belay loop on his harness was in a weakened condition prior to the climb, and that they had talked about its poor condition three days earlier. ... .
[Submitted by Keith Lober, Emergency Services Coordinator]
I think it's the exception that proves the rule though. Word when it happened was he'd been cleaning loose rock on rap, and was not only wearing a blown-out harness (with a replacement on order at the time) but also apparently had a haul bag full of rocks clipped off to himself.
So -- if you are one of the world's most prolific climbers, use gear you have already decided to retire, and have an unusually high working load, you can maybe get your gear to fail. But it's still incredibly rare.
He was a widely known and well-regarded big wall guy who had freed gigantic routes in Pakistan, Greenland and the Yukon. I read his trip reports with awe. I would have felt out of place carrying his rack from the truck to a picnic table, much less talking climbing with him.
I had not heard that he was tidying up. He was also reputed to be very generous with his time and expertise. I find the idea of him carefully cleaning up a new route (while passing an increasing load through a frayed harness) almost intolerably painful to think about.