Revealing mistake: In the shot of De Niro sitting in the plane you can see the controls next to the pilot. We hear the plane start but the pilot never touches the controls. It taxis for takeoff and the mixture controls (red knobs) are still in the mixture cutoff zone. No fuel would go to the engines and they would not run. Also both engines start at the same time, which is impossible with any twin engine airplane. Not to mention that the avionics are all dark as well and the engine gauges read zero. (02:35:00 - 02:35:40)
Revealing mistake: When Frank kicks the grocery store owner in the face twice his kicks miss by far.
Chosen answer: First off his name was Chuckie O'Brian. Jimmy Hoffa wasn't his biological father, he was more in the nature of a foster father. Second, he probably knew what happened. What was Chuckie going to do? It was a sanctioned hit. And if he went to the police he would have ended up the same as Jimmy Hoffa. They put him in the middle of it to keep him quiet. If he goes after the people who did it, he's as good as dead. If he talks he could end up in jail with them, and the mafia will probably kill him anyway.