Continuity mistake: When Nicky goes to Sam's house to meet with Charlie, it's very light outside when he arrives. However when he leaves about four minutes later, the sun is setting. His entire stay at the house takes place during that scene.
Character mistake: When the high roller Ikachawa returns to The Tangiers, Rothstein said he's betting $1,000 instead of his usual $30,000, so when he won, it didn't feel like he had won $10,000 (10 $1,000 bets), but felt like he lost $90,000, alluding to the fact that betting 1/30th of his normal bets made him feel even though he was winning $1k a hand he was missing out on the other $29k, or losing that. At ten hands won, at $1k/hand, he really would have felt like he was losing $290,000, not $90,000 as Rothstein says.
Continuity mistake: About an hour and a half through the movie there is a scene with Nikki on a black and white TV. On the TV they are showing a photo taken by police of Nikki talking to another man holding a toothpick to his mouth to cover his lip from lip readers. That scene takes place later in the movie. The police have not yet taken that photo, we see it happen later on. (01:32:49)
Continuity mistake: When Sam is talking with Philip Greene after firing Ward, there is a shot of Sam facing the camera gesturing with his right hand not holding a cigarette, the a shot with his rear to the camera holding one, back to a shot facing the camera with empty hands again. (01:10:45)
Other mistake: When the man goes into the casino and takes a suitcase full of money to Kansas, he is parking in a supermarket. Later in the movie, after a plane lands on the golf course the same shot of the car in the supermarket is repeated. Check for the same parked cars: a blue VW beetle and a red car.
Audio problem: When Nicky is at the airport having his suitcase searched by Customs, he says "If you find any cash in there we'll whack it up with ya" but his lips don't move in unison with the sentence. (00:27:50)
Audio problem: When Pat Webb is reading aloud the newspaper article about Sam Rothstein, his lips don't match when he begins the sentence reading "the Midwest bookmaker", and also the audio sounds spliced in. (01:31:15)
Factual error: About a half hour into the movie, Nicky goes into the sports book to collect. There is a cigarette machine behind him and the price of a pack of cigarettes is marked $2.50 In the 70's a pack of cigarettes was more like one dollar. (00:33:04 - 00:33:35)
Character mistake: At their wedding reception when Ginger is talking to Lester on the phone he reminds her of when he watched her as a young "long legged colt." As a juvenile female horse she would have been a long legged filly.
Other mistake: When Ace meets with Lester in the diner, Lester looks around at the guys guarding the exits. The camera pans around to show his perspective, looking first at the door directly behind him, and then the door to his right. Except the camera swings left for both shots, meaning Lester would've had to crane his neck around 270° to see the right-hand door while looking to the left.
Continuity mistake: After the very tense encounter between Ace and Nicky in the middle of the desert with De Niro watching Pesci leaving quickly, the distance between Ace and his car changes enormously in two different shots.