Revealing mistake: In the scene where Rocco kills the two 'goodfellas' in the restaurant, he hides his gun by holding the gun behind his coat. Before the shot switches, you see the gun go off, yet the hammer is down the entire time. The revolver could have been single or double action, but the hammer would still have to have gone back. It is easier to see the gun go off on the NC-17 version or on the deleted scene, but you do see the flash on the R version.
Revealing mistake: Prior to the scene where the trio raids the card game, the three bring the wife, hands bound, to enter the security code. When the guy "cuts" the duct tape, look closely at the wife's hands, the knife never actually cuts the tape; rather he pretends to cut and simply lifts the tape... and voila, her hands are free.