Revealing mistake: Near the end when Ace gets into his Cadillac, in the first shot facing the front of the car, there is a protective glass in front of Ace (the same width as the windshield) with the dashboard in between that glass and the windshield. Note the vertical edge of the glass is visible at the right side, and the rearview mirror's sticker on the windshield casts a shadow onto the second glass. During the stunt, the flames on the dashboard are between the glass shield and the windshield, keeping the actor safe from harm. (02:46:05)
Revealing mistake: At the beginning of the film where Robert De Niro gets into his car and it explodes, you can clearly see the cut where they have replaced him with a dummy. (00:00:50)
Revealing mistake: When Sharon Stone gets thrown out of Joe Pesci's club, you can see the pads on Stone's hands to break the impact of the fall.
Revealing mistake: In the scene where Joe Pesci is beaten in the corn fields, if you look closely at the bats you will see them bend ridiculously on impact with his body. Foam bats anyone?
Revealing mistake: In the scene where Pesci's character is being buried alive, the body is quite obviously a fake: look at the mechanical way the head moves, and the movement of the chest intended to simulate breathing, not very convincingly.
Revealing mistake: When Nance comes stumbling out of the Costa Rican house holding his stomach, you can tell he is holding a mouth full of blood that he spits out when he gets shot in the head.
Answer: It's also possible that the "bosses" would be concerned that a rift between Nicky and Sam could bring public (police) attention to the criminal activities going on behind the scenes. These are, after all, two very egotistical individuals.