I was trying to remove my pedals once and slipped causing my hand to slide across the large chainring very quickly. Didn't feel anything at first, but then the pain slowly got worse as the air hit it. Then I saw a pool of blood froming on the floor. Not quite bad enough for stitches, but it was a close call.
I now always put the chain on the large chainring when doing work like this to prevent any "accidents". It'll still hurt if you hit the chainring, but at least with the chain there you aren't going to slice your hand open like a knife through butter.