When the soldiers push the VW into the moat, the passenger side window is open. The car sinks and then pops to the surface and floats (air tight and they really do float) ... but when it surfaces, the passenger window is up. Of course, the car would have stayed on the bottom, had the window been open.
Just prior to riding to the summer house, the major was on his horse and had his gold leaf on his helmet. When dismounted and examining the ground for tracks, the majors' leaf was missing.
In a very typical mistake for the period, the "German" tanks are all ex-Soviet T-34-85. German tanks were simply unavailable, but no attempts were made other than painting them Grey, which was also incorrect for the period.
The major switches uniform types during the movie. From regular infantry and then later to Airborne. This wouldn't happen. Airborne uniforms were limited to airborne troops and would not be available to others.
The bazooka they use is a Korean war era 3.5" version and not the WW II version. Also, the .30 and .50 cal ammo boxes seen occasionally were Viet Nam era designs and not WW II designs.
During the retreat sequence through the town, many of the GIs are not in standard American uniforms. As the film was shot in Serbia, many of them apparently have on Yugoslavian camouflage uniforms.
The soldiers refer to the car as a "Volkswagen", a name which some viewers believe was not given to the car until after World War Two. However, on 28 May 1937, the Gesellschaft zur Vorbereitung des Deutschen Volkswagens mbH was established by the Deutsche Arbeitsfront. It was later renamed "Volkswagenwerk GmbH" on 16 September 1938. It is not clear whether the GI's would have easily recognized the car, but any German would have readily recognized it as a Volkswagen during the war.
When the tank pursues the bazooka team into the church, a close-up of the bazooka's muzzle immediately before it fires clearly shows that the tube is hollow and the weapon is therefore not loaded.
When Maj. Falconer looks through binoculars, it shows the 'double lens" movie look, even though he only has one eye.
About ten minutes from the end when the Germans are approaching the castle moat gas nozzles can be seen bottom left providing the flames. They are not quite obscured by the bushes place to hide them.
Although set during WW2, almost all the characters (including the women) wear 1960s hairstyles. Regulation army haircuts would never have permitted sideburns or hair long enough to touch the collar or go past the ears.
During the final battle, Sgt. Rossi appears to be using a M-60 machine gun. This weapon was not created until 1957, and not issued to units until 1959. Therefore, this weapon was never used in WWII.
U.S. armed forces were racially segregated until 1948. However, the source novel explains that the group was actually an ad hoc advance party for a medical unit that was going to move into the area. As such, it was made up of diverse walking wounded personnel from different units.
When the German light plane is shot down by Captain Beckman, the smoke is clearly coming from smoke canisters attached to its wheel struts and not from any other part of the plane.
Despite the fact that the Americans capture a German tank and drive away in it; although how they did so after hitting with a bazooka is another point; the tank is never used in the defense of the castle.
During the defense of the castle, numerous fences of barbed wire strung as obstacles can be seen. As the group originally arrives in a jeep, it is never explained where they obtained the materials for such elaborate defenses.