Continuity mistake: In the scene where the boys are changing into all their hunting gear on the side of the road, keep an eye on Christopher Walken. In a wide shot, he's pulling on a dark green sweater. There's a quick shot of De Niro, then back to the wide shot, in which Walken is again pulling on the dark green sweater.
Continuity mistake: After Michael returns from Vietnam for the first time, he goes for a walk with Linda. As they start to pass in front of a store with a guy outside, Michael is shown fiddling with the collar of his coat. The shot changes, and though from their position and reaction of the guy outside it is supposed to be a direct continuation, Michael suddenly has his hands buried deep in the pockets of his coat.
Continuity mistake: When the car vs. lorry race occurs near the beginning of the film, as the car pulls in front the left wing of the bumper clips the lorry and gets noticeably misshapen. Then the shot changes, the car spins round, comes to a halt, and the damage has gone (and remains so for the rest of the film). Possibly made worse by the fact they then make a big thing of Nick actually getting out the car and inspecting it for damage in a disbelieving fashion that anyone could have pulled it off without damaging it.