When Michael comes back from Vietnam he has a full beard while in uniform. He would not have been able to leave Vietnam in uniform until haircut and facial hair complies with uniform regulation AR 670-1.
In the first hunting scene as they're changing their clothes, Michael and Stan are arguing about Mike's boots. Nicky is seen putting on a sweater and had both arms in when it cuts back to Mike who delivers his "This is this" lines. When it cuts back to Stan and Nicky, Nicky is once again putting the sweater on as if it was never on in the first place.
Near the end of the movie, right before Nick's funeral, the scene shows the Eagle Super Market with the exact same sale signs in the window as at the beginning of the movie, which were different when Michael had returned home from Vietnam, some year-and-a-half later.
In the scene at the VFW hall where they are playing bingo, the guy calling out the numbers on the winning card calls B-15, but when the board comes into view showing the numbers that had been called, B-15 is not lit up.
In the bar before Nick, Steve and Michael are going off to Vietnam, Nick opens up a can of beer that he shook up, spraying its contents. In the next scene, Nick is opening the same can of beer a second time.
In the bar, right before the guys start singing along to Frankie Valli's "Can't Take My Eyes Off You", Michael is sitting on a chair holding a pool cue in his right hand and tapping it on the floor. A split second later as they begin to sing, he pounds the cue stick on the floor with his left hand.
The hunting scenes supposedly taking place in the Appalachian Range are obviously filmed elsewhere: The Appalachians are a smooth rolling range, but the scenes show towering, jagged peaks. The scenes were filmed in the Cascade Range, clear across the country.
Mike wears the green beret of the U.S. Army Special Forces operator. He is dressed in SF tigerstripe camouflage in Vietnam. However, in full dress uniform his left shoulder sports the 101st Airborne Division badge.
In the hunting sequences Michael (Robert De Niro) carries a left handed Winchester Model 70 rifle, with the bolt handle on the left side. However in one shot during the second hunting scene, the bolt handle is suddenly on the right hand side. His wristwatch is also on the opposite wrist. The editors apparently flipped the negative so that he would be facing the right direction to match the other shots in the sequence.
When Linda goes to Mike and Nick's trailer for the first time, the trailers hitch as well as the tail lights can be seen. The wires from the tail lights are seen running into the windows above the hitch. The tail lights should be on the rear while the hitch is on the front.
The goof item below may give away important plot points.
When Nick is killed in the final Russian Roulette game, the gunshot is underneath the flow of blood on his bandana, yet on the close up, the gunshot is near the top of his bandana while the flow of blood is underneath.