At one point in the court case, the bailiff is asked to remove the jury from the courtroom. The door to the courtroom is heard closing off camera, just a split second after several jury members are shown to still be in the courtroom.
When Mick arrives at his house at night and the door is partially open, it closes from the right (door knob would be on the left outside, and right from the inside). When he goes to close it after the brief meeting, it closes from the left.
The answering machine message from Frank that Mick plays after his death is different the two different times he plays it back. The first time, Frank doesn't mention the parking tickets, the second time, he does.
When Frank is murdered, the police identify the make and model of the gun simply by having a cartridge case, which is impossible most of the time. One detective alludes to having historical evidence from the gun, but that supposes their ID of it is correct.