When I was younger and riding my BMX bike, someone mentioned that my chain looked loose while I was riding. I reached down to test the tension with my fingers and rather than freewheel back to move the right pedal, I pedaled forward. Unfortunately, I had not released my grip on the chain so two of my fingers went between the chain and sprocket. I suffered giant punctures through both of my fingers/fingernails - they were mangled for weeks. That hurt bad!