The professor's answer key for the exam, given to you by the professor, is not another test to be corrected.
If, by some chance, the professor's answer key appears to have a mistake in it, do not mark it wrong.
And if you do choose to "grade" the professor's answer key, and mark his or her answer wrong, do not then go on to blithely mark all 150+ of the students who got the same answer as the professor's key wrong.
The correct response to this situation is to find the professor, and say, "Excuse me, but I noticed this and it seemed to be wrong. Is this a typo?"
The correct response is NOT to take the initiative to assume that the professor who has taught this course for decades is wrong and proceed to mark one hundred and fifty plus freaking tests wrong without bothering to check.
I hope this clears up some issues in expectations for a TA.