I have a simple humanoid armature: no IK rigging yet and it worked fine until the arms started messing up when I try to move them. As far as I can tell there is nothing wrong with the weight painting, I even cleared all the weights off of all the vertex groups in the arms but when I repainted them the same thing happened. The mesh doesn't follow the bones at the same speed and deforms wrong and I have no idea why or how to fix it.
The only modifier I have active is the armature so its not a stack order problem, and none of the other bones have any weight on any part of the arm so I really just have no clue what is wrong here and any help would be greatly appreciated.
EDIT: Here is the .blend file