Towards the close of the film, we see a commuter train at a railway crossing - whilst the train is clearly electricity powered (pantograph and overhead power cables are visible), we hear the sound of a diesel locomotive.
When Haller comes home and finds his door ajar, the door opens inwards with the hinges on the left hand side (as viewed from inside the room). He then confronts Roulet and escorts him from the house, but now the door hinges are on the opposite side.
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.
As Haller is leaving the court after Roulet's arraignment, he tells Dobbs they'll meet at Dobb's office at 4 pm. When Roulet approaches Haller and Levin in the lobby of the law office, Haller says "Good Morning" not "Good Afternoon."
At the end of the movie, Louis Roulet's mother gains entry into Mick's house without having to break in by force. This would mean that she simply used the key given to her by Louis -- except Mick would have certainly changed the locks after Louis got in the first time.
The goof items below may give away important plot points.
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.