Back in grade school i was on this Cub Scout camping trip. It was your standard group camping trip sort of outing, in that to make up for there being no rain it got so cold that little ice balls formed on the outsides of all the tents. Anyway, that next morning, near the time we'd all be packing up and heading home, all the kids were gathering these huge nuts from some common species of tree in the area to bust open in various different ways. Which mostly meant "pry open with your knife." Anyway, i joined in, as little kids are apt to do, and went to work on my first nut.
Not two seconds later, i'd split nearly my entire thumbnail down the middle because in trying to twist the knife between the two halves of the nut casing i clapped the blade closed on my hand. Needless to say, it was very bloody and i shouted and yelled and cried and generally carried on and it hurt like crazy.
So yeah. Moral of the story? Get pocketknives with lockable blades.