Plot hole: *SPOILER* Toward the end of the movie, Ryan Gosling goes to Hopkins' house where Hopkins is tricked into not only confessing again, but giving Gosling the murder weapon, after they are back in court and Gosling is the acting prosecutor. This would be a conflict of interest due to the fact that Gosling is a witness.
Suggested correction: First, the gun that Beachum took from Crowford's house was not the murder weapon. It was Crowford's unfired gun. He only took it out of the fear of his life. Second, Beachum entered Crowford's house with police supervision. If he plays it by the book, Crowford's confession is valid. In that case, supervising officers will stand witness, along with a recording confirming their testimony. Third, Beachum doesn't need the confession anymore. He was amply clear on that matter.
Plot hole: In this film, a murderer (Mr. Crowford) goes free after recanting his confession (alleging duress) and concealing the murder weapon. The film forgets the most damning evidence: The perp and the victim had been alone in a closed room from which witnesses had heard shots fired. The perp himself establishes that he had motives. He cannot go free without an astounding alibi.