at the end of chapter 19, the reader can conclude that tom might be found guilty on what verbal mistake he made?

I think this has to do with the fact that Tom felt sorry for a white girl,

"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.

Tom feels sorry for Mayella as one human being for another, but Mr. Gilmer and others can only see a black man feeling sorry for a white woman, suggesting the uncomfortable-for-them idea that white skin doesn’t make a person automatically better off than anyone whose skin is black.

I think this has to do with the fact that Tom felt sorry for a white girl,

"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. "

Tom feels sorry for Mayella as one human being for another, but Mr. Gilmer and others can only see a black man feeling sorry for a white woman, suggesting the uncomfortable-for-them idea that white skin doesn’t make a person automatically better off than anyone whose skin is black.