Protect your pinky toe!
I was pedalling uphill and somehow my shoe got caught under the protruding obstacle. I had no idea that I could have a huge hematoma on my pinky toe because of this.
When you are so focused on the big goal while pedalling (to make it through a tough section) don't forget about little nasty "pocket-like" obstacles that are often visible only in your peripheral vision. Try to keep track of everything that protrudes less than 90 degrees on the SIDES of the trail and can catch the pedal and the front part of your shoe. It will hurt!