I do it without even thinking sometimes when I'm going through withdrawal of the razor.
Normally it's just re-opening old wounds, but I got really freaked out the other day cuz I tore up my shoulder which had no prior injury. I was ripping off flesh with my fingers and nails. For situations when I can't have blood everywhere, I tend to choke myself until my arms go slack from loss of air and I nearly pass out. It ain't pretty.