Mr. Monk Is on the Run (1) - S6-E15
Plot hole: The D.A. says that ballistics show that the bullet came from Monk's gun. The simplest thing to do would be to look at the gun, they'd know that it hadn't been fired and was full. With Monk's OCD we know it was cleaned, and the sheriff came in right after, so Monk couldn't have reloaded it. Yet they overlook all of this.