possible sprained or broken shoulder
me and dad were out riding the other day when we came up on a ditch about 2 feet deep. the angle on both sides was around 45-50 degrees. we started off trying to let our front wheel touch the bottom, then stop the bike for as long as possible, before riding back out. well, that went well, UNTIL, we decided to stop in the ditch, pull out around halfway, then roll back into the ditch. i'm sure you all know, it's kinda hard to roll backwards on a mountain bike. i lost my balance, and slammed on the REAR brakes, BIG MISTAKE. before i knew, i was in a wheelie and only falling back more. we use clipless pedals, so i couldn't just put my feet down. i began to fall to the right, while still clipped in, and SLAMMED (yes, slammed as in really hard hit) into the ground. it hurt like crazy, and still hurts when i move my shoulder into certain places, or put lots of stress on it.
and hey, why go to the doctor??? they'll just say your shoulder's broke, give you a sling (which i already have a collection of 3-4) take your money, and send ya packin'. i think it's just sprained though, and i have fallen on it twice since then, which only made it hurt more.