Two times ago I hooked my handle bar as I was going around a tree - not so good, but I survived. I rarely go an afternoon with out taking at least little tumble, but it's all soft ground and I always wear my helmet.
But with that said...the MOST challenging part of mountain biking for me is the aerobic aspect. I'm huffing and puffing the entire time, and really sucking air on the up-hills. Why is that so dang much fun???
I'm heading out tomorrow night for ride! Stay vertical cats!