(In a vicious attack of irony -- as someone on MountainProject said, this roped stuff's dangerous, maybe he should stay away from it for a while ...)
At least it was mild enough that he wrote his own accident report for the American Alpine Club:
Fall on Rock – Lowering Errors, Rope Too Short
Author: Alex Honnold
I had run up the route Godzilla (5.9) to put up a top-rope for my girlfriend and her family. At the last second her parents asked us to hang their rope instead of ours. I didn't think about it, but their rope was a 60m and mine was a 70m. I was climbing in approach shoes and everyone was chatting at the base—super casual, very relaxed. As I was lowering, we ran out of rope a few meters above the ground and my belayer accidentally let the end of the rope run through her brake hand and belay device. I dropped a few meters onto pretty gnarly rocks, landing on my butt and side and injuring my back a bit (compression fracture of two vertebrae).
Lots of things should have been done better—we should have thought about how long the rope was, we should have been paying more attention, we should have had a knot in the end of the rope. I wasn't wearing a helmet and was lucky to not injure my head—had I landed on my head, it probably would have been disastrous. My belayer had been climbing less than a year. Basically, things were all just a bit too lax. (Source: Alex Honnold.)
IIRC, Dave MacLeod was injured in a very similar way, so this has to go on the tally of reminders (along with Lynn Hill and John Long decking after failing to complete their knots) that no-one's too experienced to get bitten by an error.
UKClimbing has an article on 8 Common Climbing Accidents And How to Avoid Them
Look after yourselves out there.