Deliberate mistake: When the two assassins enter Leo's bedroom and try to gun him down, the one on the right has left his tan overcoat open. He's the one that escapes from Leo initially. Shortly thereafter, Leo guns him down, firing up into a second story window. At this point we see that the assassin's coat is now buttoned up, probably to conceal the numerous squibs used in his death scene.
Continuity mistake: When Bernie pays a visit to Tom after he decided not to kill him, Bernie has a pistol lying on the small table next to the armchair he is sitting on. When Bernie is talking first the gun is white, but when the shot changes to Tom and back to Bernie the pistol somehow turns into a black one.