Plot hole: If Walter was indeed immortal from his deal with the devil then there was no reason for him to use the escape clause when he was sent to jail. Since he can't be killed by any means and nobody can harm him there would have been nothing to stop him from escaping the prison the moment his cell was opened.
Continuity mistake: When Cadwallader puts a seal on the Escape Clause paper, he presses so hard that he manages to lift the whole document with seal. You can see that there is absolutely nothing except for a seal mark on the paper, yet when he drops the paper to the floor some typed text appears on it. Also in the same scene, the seal pattern is square in the first shot, but when the paper drops to the floor the seal mark is round.