Another more figurative injustice - and perhaps the greatest crime in the novel - is Bob Ewell's attack on Jem and Scout. 

This is not an injustice of the court system, it is a moral injustice when Ewell attacks the children of the man who was only doing his job and telling the truth in court. Instead of attacking Finch himself, Ewell attacks his children, who are young and had nothing to do with the trial.

The two major injustices are the treatment of Boo Radley and the accusing and convicting of Tom Robinson.  Both were gentle, kind men who did not deserve what happened to them.  The larger injustice is that people don't accept anyone who is different.

Another major injustice was the way in which Boo was treated by his father. Arthur Jr. was apparently a normal, well-adjusted teenager when he was arrested, but his father's unusual punishment--to lock him away inside the Radley House with no contact with the outside word--turned Arthur into the "malevolent phantom" Boo, who only came out at night. Even following the deaths of Boo's parents, Boo remained a recluse within his own home, where his mental state must have continued to deteriorate as the years passed by. Sadly, Boo's heroic rescue of the children on Halloween night did not change his unusual life, since Scout "never saw him again" after that night.

I'd agree with Post 2 that Tom's conviction is the greatest specific example of an injustice. But I think we'd also have to look at the pervasive racism in Maycomb as an injustice. The ingrained attitude of so many people makes it possible for hatred and prejudice to flourish in society. So, while Tom is obviously discriminated against, his fate is made possible by society's long held belief that one kind of person is superior to another kind of person.