In the scene where Ace (Robert De Niro) and the rest of the casino staff capture the first of a pair of cheaters (a bearded man in a white shirt), move him to a back room, pull his pants down and place both his hands on the table, observe the placement of the guard's hand holding the cheater's right arm.
At first you can see the guard's hand holding the cheater near the elbow, then we a shot of Ace, then when the camera turns back to the cheater, we see the guard holding the cheater at the wrist. Then after another quick shot of Ace (where we see no movement from the guard), the next shot on the cheater shows the guard holding the former at the elbow again.
- no drastic change in hand placement occured with the guard holding the left arm
- the right arm (particularly the hand) was the one hammered, so the slip up could be due to the placement of a fake arm