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.)