Unexpected red stuff!

I’ve just had a very strange experience. I started typing, I realized that there was something soggy on one of my fingers and I saw red stuff on the “s”, “w” and “ctrl” keys. Then I spotted a small, front-to-back slit in the top of the third finger of my left hand. The puzzling thing is how I managed to cut myself between switching the computer on, letting it boot up and starting to type something given that there’s nothing with an obviously sharp edge anywhere in sight. It’s the sort of thing which would give my health & safety inspector nightmares – if I had one.