When Are You Owed an Apology?

I find it fascinating that in the light of the political dust-up about Mitt Romney's work and filed forms about Bain Capital that Romney has demanded an apology from the President.  It leads to the general question about the grounds that would qualify one for an apology. 

In this case, you have a presidential candidate having his record challenged.  That seems par for the course.  If you are running for President of the United States, you know -- or should know -- that this sort of scrutiny of your history and its comparison to your claims on the campaign trail will be made and portrayed by your opponent in a light that favors him./her and not you.  That is what happens in campaigns.  If one of my kids were to punch the other in the nose, before the talk of consequences began, the one who threw the punch would make an apology.  But for a professional boxer to demand one from his opponent for punching him in the face seems absurd.  A legitimate demand by person A for an apology from person B therefore seems to require not only that person A was harmed by person B's actions or words, but that person A did not willingly put him/herself in a position where the harm was to be expected.

This seems to be a sort of social contract-based justification.  Apologies are required when a well-understood line is crossed.  But does there have to be such an understanding on the part of both people?  Suppose they have different senses of where the line between acceptable behavior and unwarranted attack lays.  Could one demand an apology for not only the behavior, but for not being thoughtful enough about what makes an act allowable in the context?

What are the grounds for legitimately demanding an apology?