Mesh deforms when importing animated fbx from Blender

When I import a humanoid fbx with animations Unity (perhaps Mecanim) turns the models upper arms into twisting noodles. When I first noticed this I thought it was being caused by my model having a resting A-pose that Mecanim forced into a T-Pose. So I went and changed the resting pose into a T-pose and redid all the animations. Unfortunately, that doesn’t appear to have been the problem, because Unity still turns the models arms into noodles.

Are there extra bones in between the mecanim assigned humanoid bones?

This may also be caused by not restricting the maximum vertex influences in blender to 4. In Unity the maximum number of bones that can influences one vertex is 4. If blender is not set to this - the skin weights might be influenced by more than 4 bones causing skinning anomalies in Unity.