Did you think drinking your own urine was bad? To truly test whether you have the right stuff, imagine ripping out your own fingernails, on purpose. A couple of astronauts have done this before going into orbit, because they figure it’s better than losing them inside chafing, unwieldy spacesuit gloves, according to Dava Newman, director of MIT’s technology and policy program and director of the university’s Man Vehicle Lab.
Astronauts with large hands are more prone to suffering from this complaint as their gloves are too tight. This cuts off circulation and causes fingernails to fall off. Engineers are currently working on a design for gloves with robotic joints to address the problem. I'll put off space travel until there is a fix.
http://www.popsci.com/science/article/2010-09/bulky-tight-fitting-gloves-cause-tough-nails-astronauts-lose-their-fingernails - Via Quiddity