Mistakes Made by the Defense

It's not so much what the legal team for Leo Frank did, but what they didn't do during the trial:

1. NOT CROSS-EXAMINING THE FACTORY GIRLS WHO TESTIFIED AGAINST FRANK
Many of these girls claimed that Frank made "improper advances" toward them which relates to the prosecution's theory of how the murder occurred. It is likely that many of these claims were either completely false or embellished. Had Frank's lawyers probed into them, it might have put doubt in the juror's minds. Their excuse that the jury saw each witness as a replica of the victim played right into the prosecution's hands.

2. NOT REQUESTING A CHANGE OF VENUE
If the atmosphere in and around the courthouse was as bad as we are led to believe, they should have asked the judge if the trial could be relocated. The defense lawyers surely knew that the laws of Georgia would allow for it due to circumstances. They could have at least discussed this with Frank and if he was in favor, then spoke with the judge. This is also mentioned by Governor Slaton when he decided to commute Frank's sentence.

3. NOT BEING ABLE TO "BREAK" JIM CONLEY
This is the most important of all. Conley was coached by his attorney and given a makeover to look respectable so the jury would believe him. As hard as they tried, Frank's lawyers could not get Conley to contradict himself even though he had previously given four differing affidavits to the police and admitted to lying during his interrogation. The most ironic thing, which is not depicted in the film, was the attorney that coached him for the trial would later conclude that Conley committed the murder and then support the cause for Leo Frank.

There's no guarantee that if Frank's lawyers had done these things he would have been acquitted, but if the jury would have been in doubt, it might have had an effect on the verdict and the ordeal Frank suffered.