In Harper Lee's To Kill a Mockingbird, the most significant moment demonstrating the cruel and unfair treatment of Tom Robinson occurs during Mayella Ewell's testimony in the courthouse.
During her testimony, Atticus asks her to identify "the man who raped [her]" (Ch. 18). When Mayella points to Robinson, Atticus has Robinson stand, asking Mayella to "have a good long look" at him. As Robinson stands before the court, we see that his left arm is much shorter than his right and hangs "dead at his side"; his left hand is also shriveled--all as a result of his having had it caught in a cotton gin as a child (Ch. 18). Prior to Robinson standing, both Sheriff Heck Tate and Mr. Ewell had testified that the right side of Mayella's face had been bruised, and such bruising could have only been accomplished by a man's left hand when facing her. Robinson's crippled arm, coupled with Atticus's ability to prove Mr. Ewell is left-handed, shows beyond a doubt that Robinson is innocent, and Ewell is the true guilty party. Yet, despite Robinson's proven innocence, the jury still returns a guilty verdict and has Robinson sentenced to death. It is very clear that the jury returned the guilty verdict all due to the color of Robinson's skin, as we see when Reverend Sykes declares to Jem, "I ain't ever seen any jury decide in favor of a colored man over a white man" (Ch. 21).
Prior to the trial, Robinson is treated cruelly and unfairly when a lynch mob led by Walter Cunningham, Sr., attempts to storm the jail to bring their own sense of justice. Prior to the arrival of the mob at the jail on Sunday night, a different mob led by Sheriff Heck Tate goes to Atticus's house on Saturday night to warn him and question his sense in taking the case. When Atticus asserts that his job is to let the truth of Robinson's case be known, they begin to move in on Atticus. On Sunday night, the mob led by Cunningham arrives at the jail and is blocked by Atticus. Luckily, the mob breaks up when Scout reminds Cunningham of his sense of humanity.