Continuity mistake: Near the end, when the hitman walks out of the diner, the camera focuses on the car in the parking lot, cuts back to the hitman, then cuts back to the car. The problem is the shadows in the first shot of the car are about half a meter long, but a few seconds later they are two meters long. Obviously both takes of the car were taken hours apart.
Continuity mistake: When DeVito leaves the bar and goes towards the bathroom, in the shot from behind he has almost reached the arch when the shot ends. In the next shot he is almost back at the bar, and it takes a while until we can see him passing through the arch. That's impossible, even though the second shot is a bit more "close up".
Continuity mistake: At the beginning of the conversation between Hoffa and the Detroit newspaper journalist you can see Hoffa sitting in the car, turned towards the outside (talking to him), his shoulders almost parallel to the door frame. In the next shot he is sitting immediately parallel to the seat.
Continuity mistake: When the assassins close the door of the truck, the rope used to pull down the door is still tied around a handle on the right side. A shot later (when the truck is on the road) the rope is gone.