I once came home to find my baby Naco running around the cage during the day in a panic.  After taking him out I realized there was blood all over the cage and his tail.  From what I can see it had gotten caught on the bars of the metal cage.  Poor little guy, I brought him to the vet ASAP and they had to amputate part of his tail.  He recovered just fine.  He still is in a metal cage and luckily neither him or his girlfriend Gamma have gotten their tail caught since.