Continuity mistake: In minute 5, when Nick Fisher raises his hand, his watch wasn't touching his sleeve and the time on it was 4:30 and 47 seconds. In minute 105, when the scenes were repeated, the watch is partly under his sleeve and the time is 4:32 and 40 seconds. (00:05:00 - 01:05:15)
Continuity mistake: When Slevin finishes shaving he wipes most of the shaving cream off his face and stares at himself in the mirror. There is still some shaving cream left on the underside of his chin and neck though. When there is a knock at the door the shot changes and the shaving cream has vanished.
Factual error: In the parking lot of the race track, as the camera zooms out you can see a new Ford Thunderbird. This is impossible, as the scene takes place in 1979.