Last night, I was power carving a bowl with my Foredom and started to change the carving bit. I had the wrench attached and was just about to insert the locking pin when I accidentally stepped right down on the footpad. The wrench on the collet started spinning instantly at about 10,000 RPM with the locking pin still attached at the other end of a chain that holds the two together. It whipped the back of my left hand many times, breaking the chain across my knuckles before I knew what happened, and hard enough to break that chain in many bits. I found pieces of it this morning all over, but still have not found the locking pin. My hand was cut in several places and quite swollen. It’s very sore, but fortunately nothing broke and it should be back to normal in a week or two.
I learned from this experience that is is definitely safer to unplug the tool before changing bits. It would have saved me time in the long run and I'd be carving tonight instead of writing this...