I was handsanding very hard and very fast and my left hand lost grip of the razor but for a moment and my other hand was of course still going hard at it and slammed the razor's edge into my thumb under the joint. I was lucky that it jammed me in a perpendicular motion. It looked like it should have been very nasty, but it wasn't deep at all and only bled for about 45 seconds. An inch long cut
That was about two months ago, there is no scar
Now on the rare occasion that I sand, I use gloves