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.
Continuity mistake: When Vinny, Tyrone and Sol are waiting to rob the bookies and the dog starts barking and biting Sol he is already reaching for the squeaky toy and even says "here", but when the camera cuts to Vinny and he tells Sol to give the dog the toy, they show Sol reaching for the toy again.
Continuity mistake: In the scene where Bad Boy Lincoln is trying to sell a fake diamond to Sol, notice the security monitor to the left, in one shot you see his hands down, and in the next shot he is rubbing his head. Now in the next shot on the security monitor he his rubbing his eyebrow, then in the next shot his hands are down. (00:23:18)
Continuity mistake: During the final fight scene, the trainers from both sides run into the ring and have a scuffle, Mickey is on the ground recovering. Tommy and Turkish are in the ring fighting with the other trainers. Turkish helps Mickey back to his corner and Tommy has magically teleported from the inside of the ring to the outside of the ring helping them.
Factual error: They mess up the number of times Tony fired his Desert Eagle quite a lot, especially in the hallway shootout scene. While Boris Avi and the black crew are arguing in the hallway, Tony fires 8 shots through the wall. Desert Eagles (the .50 calibre version, which is shown) CAN hold 8, 7 in the magazine and the 8th in the chamber. But before he fires, he cocks the slide to cock the hammer. If he did that, any bullet that would be in the chamber would be ejected. None is ejected so it must have loaded the first shell in the magazine. Hence the capacity of the gun is 7. He couldn't have fired 8 shots. He then kicks down the door and walks into the hallway, slaps in another magazine and again cocks the slide. Again based on how he loads the gun there can only possibly be 7 shots. He fires 8 at Boris. (01:16:30 - 01:17:15)
Continuity mistake: When Bullet Tooth Tony is shooting at Boris in the hallway, notice the second shot he takes - no muzzle flash. All of the other shots have one.
Continuity mistake: In the scene when Turkish and Tommy are talking to Mickey before one of his fights, he doesn't have a cigarette, the camera moves onto someone else for a split second, then moves back, and Mickey has a lit cigarette in his mouth. He is wearing boxing gloves, so couldn't have held it beforehand.
Visible crew/equipment: After Avi and Co have just met with Bullet Tooth for the first time they go out and get in his car. Here you can see the boom mike reflecting in the center of the bonnet.
Other mistake: When Bullet-Tooth Tony goes to 'open the dog' in order to get the diamond out, and then Vinny confesses that he has it, the dog starts to go crazy and in one scene proceeds to pull Sol's jacket off. If you look at Bullet-Tooth Tony's face, he is laughing at how crazy the dog is (this would have been a real-life giggle because the character is quite serious at that moment in the film).
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.
Other mistake: In the start of the film, the guard is searching Franky (disguised as Mutti) with a metal detector. The detector doesn't notice anything on his chest, only near the groin. When he reveals the guns two are on the chest.