The hairstyles the men wear in the first scenes in the film, which are set in 1967, are anachronistic for their location, occupation, and socio-political orientation. While male hippies and rock musicians famously had long hair in 1967, it was unheard-of for patriotic steelworkers in Pennsylvania to wear their hair that long. Hair for that set of people is perfect for 1975, when the last scenes are set, or 1977-78, when the movie was filmed, but not for 1967.
When Michael returns from Viet Nam he is staff sergeant stripes and a chest full of ribbons. He could never have reached that rank or been awarded that many medals in a one year tour of Viet Nam. He is also wearing 6 service hash marks on his sleeve. Each hash represents 3 years of service.
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.
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, right before the guys start singing along to "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.
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.
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.
The religious ceremonies are held in a place marked Russian Orthodox Cathedral. A cathedral in any religion is the seat of a bishop, usually located in the population center of a diocese. It is unlikely that such a place would be located in a small town such as Clairton.
Nick, who is AWOL at the time, is killed in a Saigon backstreet during the final days of American presence in the city and yet Michael somehow manages to get his body across the city, into the American compound, choppered out of the city and then put on a plane and returned to the US for burial. This is simply not possible.
When hunting, Michael 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.
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.
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.