Continuity mistake: During the complete car-bomb chase sequence towards the end of the film, Michael Clayton's car is shown in one shot without its lights on. All the surrounding shots in the scene have both cars with their lights on. This is also a slightly revealing error because it shows that they actually filmed these sequences during the day using neutral density filters.
Continuity mistake: After giving Michael the police notice for Arthur's apartment, Gene takes the paper off him and asks him who asked for this. When he asks, he is holding the paper with one thumb on the back, two fingers on the front and two fingers on the back. When the camera changes angles, it is now just his thumb on the back and all fingers across the front.