The greatest injustice in the novel is the accusation and conviction of Tom Robinson. In court, Atticus shows that it is much more likely that Bob Ewell has beaten Mayella and that Bob has used this as an opportunity to ruin Tom's life. Everyone in Maycomb knows this is likely, but the jury convicts Tom anyway. In the simplest terms, the bad guy wins and the good guy goes to jail. Tom Robinson is one of the novel's "mockingbirds." He keeps to himself and helps others when he can. Note that in Chapter 10, Miss Maudie says:
"Mockingbirds don’t do one thing but make music for us to enjoy. They don’t eat up people’s gardens, don’t nest in corncribs, they don’t do one thing but sing their hearts out for us. That’s why it’s a sin to kill a mockingbird."
The injustice of Tom's conviction is one of the most significant events of the novel. It is the town's moral responsibility to protect people like Tom.
Dolphus Raymond is criticized for living with the black community. He pretends to be a drunk to give people a reason for their prejudices.
It is unfair that the Ewell children have no real shot at a decent life. They are born into a cyclically poor family with an abusive alcoholic father.
The examples of injustice in the novel have to do with racial discrimination, social class discrimination, and prejudice. There are class divisions. The Cunninghams are poorer than the Finches. The Ewells are even more destitute. And these family lineages tend to repeat in subsequent generations. This shows a lack of opportunity for upward mobility. This is the south in the 1930s. The racial segregation of Maycomb is the culture of this historical period, but this is unjust as well.