I went mountain biking for the first time in Colorado last summer (going again in August) and I crashed so many times I can't even remember. Have so many scars from those rides yet too.
One of my falls was on a downhill ride like that where it was very steep and I was feeling good about myself =), got going a little too fast, tried using the brakes a little too much, back tire spun out and threw me off the bike, luckily I was able to grab onto a pine tree branch or I would still be rolling down that hill! That really screwed up my bike.
Another crash I had was just on a regular path in the forest, saw a large tree stump in my way, thought I could just jump it, got over it but then I noticed there was a pretty significant drop off afterwards that you couldn't really tell from riding up to it and the front tire hit the ground first, and threw me over the handlebars, hit my head on a rock (thank god for helmets) and gashed my shin and elbows pretty good.
Such an adrenaline rush though. Can't wait till August!