Hell, sometimes their parents argue about it.

The best defence against this I suspect is to have a clearly communicated, unambiguous grading scheme, but this can be difficult to do in an even handed way.

I had one undergrad prof whose policy was this: All incorrect answers get 0, no exceptions. Correct answers lose points for all steps not shown the way he had demonstrated.

There wasn't a lot to argue about, but none of us felt his approach was entirely fair.