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.