Something that has happened only 3 times in my life. It started at some point before the third grade, when I was watching a movie at my friends' house. After the movie was over, we turned the lights back on and I looked down to find dried blood covering the entirety of my legs, from knees down to ankles. After I scrapped the blood off, I would find just 2 or 3 simple cuts, but the truly weird thing is: The blood had creeped above the cuts.
This happened once in Wisconsin in the way I suggested, once in Indiana, just randomly one day while I was playing Pokemon sitting in a computer chair, and once again while I was sitting at the dinner table.
I still don't have a clue how it happened...