After Arthur strangles the arms dealer and he sets the laptop to play an erotic clip, he touches the computer with his hands not wearing gloves, leaving his fingerprints on it, which would implicate him. They also both touch the door while leaving.
When Steve is in a brutal fight with Burke at Burke's home, Steve crawls toward the fireplace and grabs a brass ash shovel and smacks Burke in the knee and head. But when he starts stabbing him with it, it becomes a black iron fireplace poker.
After Arthur's first hit in the movie, the (now deceased) victim is seen floating in the water, then suddenly, with help from Bishop, swimming forward. The camera pans underneath, showing Bishop's hands around the wrists of the corpse, but when seen from above, nothing is, as probably intended, visible.
In the scene where Arthur steals the blue GTO, the car is originally shown without the Sports Appearance Package (Grey grill inserts/different rear bumper fascia). Once the camera cuts away and back to the car, it does have the Sports Appearance Package installed.
After the first hit, when Arthur jumps from the bridge from the above angle it looks like he is going to land on a barge, and in the next shot from angle below he falls a few meters in front of the barge - apparently two different shots.
During the Priest Assassination Job, the scene is supposed to be set in Chicago. There are shots of the Chicago skyline but the hotel where the action is set was in fact shot in New Orleans. Quick shots of palm trees, external building details and skyline (when the characters are fighting on the rooftop) gives the illusion away.
During Harry's escape from his office, he sticks scissors into the outlet to kill power to the floor. In reality all this would do is trip the circuit breaker the outlet is wired to, not the entire floor.
In the overview shot of the airport in Chicago, we see Continental and Delta airplanes in liveries that have not been used for at least 10 years. Also, the Continental 747 is not a model that they have flown for some time.
Towards the end of the movie, when Steve pulls out a record to play on Arthur's record player, the cover has "Jazz" written in the corner, however when the music starts, it is Schubert's Trio n#2, which is classical music.
At the ending scene at the gas station, Steve started walking to the side of the gas station after leaving the gas pump still attached to the truck. When he turns around, it shows the pump still attached to the car when he shoots it. At the very end, police are shown examining the footage and you clearly see Arthur rolling out of the truck and the gas pump falling off the truck.