Hi, there.
If you are using mental ray shader in maya like mia_material_x for example, you might be notice that when you disconnect your bump node, your mia_material_x will give you a wrong result. That's because when you disconnect your bump node, the vector value of this bump parameter does not set back to zero. Even though mental ray already have write a script to fix this issue, but sometime the issue still there if you are not using the latest version(I don't know why). To fix that, you can just use a simple mel script to set the vector value back to zero.