In To Kill A Mockingbird, what is the mistake Tom Robinson makes when he says he felt sorry for Mayella?

Tom crosses the boundaries of race and class in an extremely poignant manner.  He, the black man, is supposed to be the lowest of the low, being both black and poor.  Even though Mayella is poor, labelled as white trash, she is still that, white.  Being white puts her above being black in the social hierarchy of the South.  It's the ultimate insult that a black man, whose station in society is as low as it gets, could have the audacity to feel sorry for this white woman.  And to add insult to injury, he is right to feel sorry for her.  It reveals his character, a character which is not allowed for by the rhetoric of his opposers.