One of my former students came to me to complain about an exam he'd had in another class, that has some overlapping material with a class the student had taken from me.
He read me one of the exam questions, and outlined the solution that the other professor had been looking for. In my opinion, the question and the solution were such an oversimplification of the real problem that the "solution" was actually incorrect. The student, having discussed this topic in greater detail in my class, knew there was a problem with it also, but wasn't sure if maybe he was missing some big idea.
I encouraged him to go talk to the other professor, and see what the prof was thinking, and explain what he (the student) was thinking, etc. Just to see where the disconnect was. So, he did. The professor agreed that his solution was "not technically correct", but said, "I was just looking for a quick answer, and for back-of-the-envelope purposes, my way is fine."
Okay, maybe. (Having seen the actual question, I feel like his approach was still no good, even for rough estimates, because it was all based on some very sketchy relationships). But, it rankles me that the students were asked to come up with a ridiculous and inappropriate answer, and then penalized for trying to go about it in a better way.
But, that's the weird thing about grading. There's no recourse when the professor is wrong, or wrong-ish, and doesn't care.
This has come up once before; a student came to me in deep confusion over a subject in another class, and since I'm very familiar with that subject matter he wanted me to try to explain it to him. When I looked at the problem, I saw that the prof was using the wrong approach for that type of situation - and then I wondered what my responsibility was towards the student. I decided to explain how *I* would do it, and gave him some references, but pointed out that I wasn't teaching the class, and so it would be worth his while to know the other guy's approach (but, it bothered me a lot, because out in the real world, that approach wouldn't be right). I even, very gently, asked that prof, if he didn't mind, to explain to me why he was using that approach, and his explanation didn't make sense to me either. But I left it there because I wasn't sure if it was any of my business.
3 months ago