One of the races is a series of loops that keeps passing over the top of the mountain, so some of the more elite runners just keep doing loops until they get to 50 miles.
If they hit their head or break an ankle or just hit the wall and can't go on, we pull them off the mountain. Usually 1 or 2 per race need a ride on an ATV out of there, and every couple of years we haul someone out on a stretcher.
My boss has done 50-milers (not trail run 50 milers) and has discussed 100-milers with many folks who have done them. According to him/them, there's not really a conditioning difference between the two: hundred milers just take a little more careful work to take care of your feet, hydration, and energy intake.
I remember last year at the Rock n Roll Nashville Half/Marathon, there were something like a dozen hospitalized and a few hundred that had to receive treatment due to injuries or the extreme heat/humidity then (I don't recall seeing any ambulances this year when I did the full marathon). And having done over a dozen trail races (28K and below), that number sounds about right for injuries/rescues.
As for 50/100 milers, that's my understanding as well, that it's the feet that are the worst physical issue (blisters and chafing elsewhere), with the lack of sleep causing a huge mental block bigger than the physiological "wall" encountered around the marathon point. I've begun training for a nighttime trail marathon in August, with hopes of building up to a 50 miler in 11.5 months. Beyond that and I probably would need a good crew to sustain me for any attempt at 100 miles (I doubt I try, but 4 years ago, I didn't think a marathon was possible).
Je suis méchant.