I watched a DVD about the Appalachian Trail a couple nights ago. At one point, they focused on hikers' feet and the brutal pounding they get on the trail. Peeling blisters, raw heels, fungus-infected soles, and black toenails. They focused in on the black toenails just as the hiker flicked his big toenail off. That was obviously done for stage effect.
Every time I've gone hiking, my toenails have taken a beating. I invariably end up losing some, particularly my big toenails. It's the descents, the constant scrunching of your toes against the front of your shoes, as you pick your way down the trail, adjusting to the new center of gravity your backpack has given you.