“I’ll repeat the question,” said Atticus. “Can you read and write?” “I most positively can.” “Will you write your name and show us?” “I most positively will. How do you think I sign my relief checks?”[...] “What’s so interestin‘?” he asked. “You’re left-handed, Mr. Ewell,” said Judge Taylor.[...]Mr. Gilmer asked him one more question. “About your writing with your left hand, are you ambidextrous, Mr. Ewell?” “I most positively am not, I can use one hand good as the other. One hand good as the other,” he added, glaring at the defense table. Jem seemed to be having a quiet fit. He was pounding the balcony rail softly, and once he whispered, “We’ve got him.”

This is a section from the trial where Atticus is questioning Bob Ewell to see which hand he writes with because Tom Robinsin's left arm is crippled meaning he couldn't have hit Mayella due to her injury being on her right side. And Mr.Ewell has a past in beating on his daughter. So Atticus did that just to see if Bob was left handed, making it look like the beating came from her dad and NOT Tom. What was wrong with what had happened was that the jury didn't even care or take that into consideration when consulting whether or not Tom was guilty or not, ending in the verdict to be "guilty."  

An incident where the person who committed the crime got away with it was the Trayvon Martin case. In this case George Zimmerman had wrongfully shot and killed Trayvon Martin saying it was in self defense. There wasn't any real evidence to back up his claim, but in the end he was found not guilty.Trayvon.