I never would have guessed this, but it turns out that it was the tongue cutting iron from a large set of tongue and groove planes. I discovered this last Saturday when said iron slipped out of the mouth of the plane (due to a loose wedge) and sliced through my pants and into the top of my ankle joint, cutting a 3" gash and cutting a tendon and a few nerves. I'll spare you the pictures but I'll just tell you it was the most gruesome injury I have ever sustained. I am healing well, and quickly, thankfully.
Your endgrain is like your bellybutton. Yes, I know you have it. No, I don't want to see it.