I will relate a true story, which demonstrates the DANGER of not knowing your partner. After this incident, I tend to look a little closer before having anyone belay me--even on a top rope.
Maybe 10 years ago, I stopped by Crow Hill, for a little workout. There was a rope already setup on Crowmagnon, by a group from MIT. I got much of my early mentoring from MIT guys, and while chatting with these guys, I asked if someone would give me a belay. A guy stepped up, and I did ask him if he was familiar with belaying, ( he was using an ATC). He assured me that he was experienced.
I climbed up and after reaching the top, something in my mind told me that I better play it safe, and down climb, as far as possible. I often do this anyway, just for the extra work out. I was about 15 feet off the ground, near the bolt, and I asked the guy if he was ready, before jumping off.
He said O.K. and when I jumped off, I went crashing down to the ground-HARD. I really was quite shocked, and couldn't understand what happened. Without even getting up--- I looked up at him and asked what happened. He replied, " it burnt my hands". The guy completely let go of the rope!!!!?
In hindsight, I really ought to have reamed him out, since obviously, if I hadn't down-climbed the route, I wouldn't be here to tell this story. I thought MIT people were smart, or at least had some BRAINS! I didn't even reply to him.
I hope this guy reflected on his actions that day, and wish he was reading this now--as well. It still pisses me off, every time I think about it--which is rarely.