Early in the movie, Lt. Rennick and his driver, Sgt. Griffin, are driving in an open jeep to Rennick's new headquarters at the convent. It is clearly winter with snow on the ground and leafless trees. Yet, when they drive up to their headquarters gate, it now looks like early summer, with trees with full leaves and no snow on the ground. Then, it's back to winter again with heavy snow when Happy parachutes back into Germany.
When Happy is being chased through the train yard, he is shown running between two moving trains. In the reverse shot, there is only one train, but then in another shot from the original angle, the second train is back again.
Karl is mistakenly called a corporal. The Luftwaffe uniform that he wears both in the US POW cage and while back in Germany has the rank insignia of an "Obergefreiter", specifically three winged emblems on the collar patches, plain shoulder straps and two chevrons on his left sleeve. Also when the list is checked for his name at the bridge the rank is written down as 'Gfr' (gefreiter) The ranks of Gefreiter, Obergefreiter and Stabsgefreiter (all which were partially identified by chevrons on the sleeve) were not NCOs and had no command authority over other soldiers. They were simply grades of seniority and would be more equivalent to Private First Class (PFC) in the US military. The German rank that is the closest equivalent to Corporal is Unteroffizer. Also, Karl is wearing the medical badge on his right sleeve; Luftwaffe enlisted medical personnel wore the badge on the left sleeve, while Wehrmacht (army) wore it on the right.
The radio in the Gasthaus is a 1950s vintage Grundig. During the war, the radio, if there was one, would most likely be a Volksemphänger.
The soldier he shot had a disk around his neck. It had an eagle on the front and Gestapo and a number on the back. The real one spelled out the words secret state police or gestapo for short and a number. It was used with a chain and kept in a pocket. It was not worn around the neck.
Before Karl (AKA Happy) parachutes back into Germany, the US crewman in the aircraft says that he is glad "every time that a string of hundred pounders leave that bomb rack". The aircraft that they are in is a C47, a cargo plane, and did not have bomb racks.
The German tanks seen in the movie are in fact M24 Chaffee light tanks mocked up as panzers.
The shot of the C-47 just before Happy parachutes into Germany shows USAF markings on the fuselage not used until after WWII.
The parachutist was issued a shovel but the military term is entrenching tool.
Happy crosses the Rhine on a bridge, going from the east side of the river to the west, since he started from Wuerzburg, which is in the eastern part of Germany and was heading west to Mannheim. Mannheim, however, is on the east side of the Rhine. Therefore, Happy is crossing to the west, the wrong side of the Rhine.
When Karl Maurer--"Happy"--is stopped at the river bridge checkpoint in Mannheim, he says he "lost" his papers, and then makes a run for it. One of the building through which he is being chased, however, is the former Bavarian War Museum in Munich, several hundred kilometers away from Mannheim.
When Happy and the Gestapo agent disguised as a soldier are being strafed by P-47s beside a river. The supports behind them are of the Mangfall Bridge, a structure destroyed during the war and not rebuilt until 1959. The Mangall Bridge is near Weyarn, which is southeast of Munich, considerably off the track from Wuerzburg to Mannheim.