As with most injuries to the head, there was a lot of blood at first but it soon stopped. I don't think it needs stitches.
As for how it happened...
I ordered myself one of those personalised embossers to use on books, so that when I loan them to students (or, more importantly, members of staff) they remember where they came from. I'll be receiving it formally at Christmas, but I thought I'd better check that it worked first.
The embosser looks a bit like a stapler. I tried it on a random piece of paper and couldn't make a dent in it. I tried again, pressing harder, and it still did nothing. I put it on my desk, stood over it, put both hands on it and pressed with my full strength.
The embosser slipped over, I followed through, and in the process banged my head on a shelf. The result is the impressive cut shown in the photo.
The embosser still left no mark on the paper, other than a fold where it had slipped. I had to take the die out of the embosser and close it onto a piece of paper in a vice to get it to leave an imprint. It probably just needs a few more of those to get the edges rounded a bit and then it'll work in the stapler-like bit.
I'm hoping that the cut heals up as well as the vertical one I gave myself in 2022 did.