Revealing mistake: Brad Pitt's tattoos appear to smudge during the last fight.
Revealing mistake: In the first 'LONDON' scene Turkish discovers Tommy has a gun. Turkish has a bottle of milk in his right hand and when they're talking about the gun, Turkish reaches for it. If you watch carefully here, he subtly passes the bottle off screen to someone so he can use both hands when handling the gun.
Revealing mistake: When Bullet-tooth Tony fires his Desert Eagle into the wall of the pub to hit the guys in the adjacent hallway, note how his pistol has no recoil or "kick" whatsoever. The .50 caliber Desert Eagle has a violent recoil and the absence of it reveals that he is firing blanks.
Revealing mistake: During Cousin Avi's 'travel sequence' there's a quick shot of him downing some medication with a drink (it occurs right before the shot of the Concord screaming towards the camera). If you look closely it's clear the glass he's drinking from is empty.
Revealing mistake: When Bullet Tooth Tony shoots Boris the Blade in the pub, the prop gun misfires on the second shot - there is no flash but we hear two shots and something falls out of the barrel revealing it to be a prop gun.
Answer: It saves time, and it's a little twisted. Would you chase after a guy who you just saw cut off a dead guy's hand to get a briefcase, then just decided to take the hand with him?