Every now and then while moving my characters legs will break for no apparent reason.

Just now I was standing on a completely flat surface indoors, I turned and walked over to pick an item up and BAM! Legs broke. Pants ruined, everything in them ruined. I was not hit by a zombie or a player.

I've had this happen twice before. Might want to take a look at how the falling damage is calculated, may be a bug in there somewhere.