What was the "mistake" Tom Robinson made in his testimony?

The mistake Tom Robinson made while being questioned by Mr. Gilmer was to admit he helped Mayella because he felt sorry for her. It was not acceptable for a "black" man to feel sorry for her, to feel sorry for her meant that Tom believed he was better than she was. This is evident in Mr. Gilmer's response.

"Yes suh. I felt right sorry for her; she seemed to try more'n the rest of 'em-"

"You felt sorry for her, you felt sorry for her?" Mr. Gilmer seemed ready to rise to the ceiling.

The witness realized his mistake and shifted uncomfortably in the chair. But the damage was done. Below us, nobody liked Tom Robinson's answer. Mr. Gilmer paused a long time to let it sink in.