I did something Saturday I recommend you never try yourself. While reconverting my Cross-Check from singlespeed for the MN State CX Championships back to fixed gear for commuting, I got my thumb caught between the fixed cog and chain. It wasn't bad, thankfully. No blood even, just a HELL of a lot of pain. I think what saved me was the rag I was using to wipe the chain was wrapped above my thumbnail and below the fleshy pad, so I had no direct steel-to-flesh contact. I also had just installed a brand new cog, not a worn one with chain-honed, sharky teeth.
I clean my fixed gear drivetrains frequently and am well aware of the danger of a stuck digit. Sheldon Brown (MHRIP) has some graphic proof of such mishaps. They ain't pretty pictures, be warned. To my credit I was following my usual, time-tested procedure -- reverse pedaling with rag right in front of the cog, allowing plenty of distance between fingers and chainring. The difference was I recall letting the rag flap about quite a lot. Somehow it got caught somewhere and reversed the drivetrain, tugging my thumb into the works momentarily. The thundering voice of Cog spoke: Keep thine rag in check.
Enough of the PSA. My efforts meant I pedaled to work on a crisp, quiet drivetrain this morning. The sun made an appearance. It was about 32F and quite pleasant. After 20s and even teens last week I am happy to have a slight upturn in the mercury.