Don't forget wet leaves. I was riding my bike well after it had rained, and most of the path was dry, but there was a shady spot under a tree that still was wet and I basically lost all traction on it. Only time I ever "used" my helmet.
Or half buried campfire rings (aka a repurposed truck wheel rim) hidden by the grass on the side of a hill.. I think I must have done about two flips in the air after hitting that. At least in that case I don't think I hit my head hard, I landed in the same grass which cushioned my landing, but if there had been something else hidden in the grass I might have lost more than just a bicycle wheel.
