In "To Kill a Mockingbird," how had Tom Robinson ever been in trouble with the law before?

Tom Robinson was once in a fight with another man, and served thirty days for disorderly conduct. As Tom describes, the other man cut Tom in the shoulder. Both men were convicted of the crime, however, the other man was able to pay the fine and did not have to serve any time. Tom could not pay and served thirty days in jail.

The significance of this information being brought up by Atticus at the start of Tom's testimony reinforces the idea that Tom has nothing to hide, that he has done nothing wrong.