After Gunnar (Errol Flynn) says to Karen (Ann Sheridan), "When my father was my age, he already had two children", there's an obvious jump cut indicating some dialogue had been deleted; this likely was due to a failure to gain acceptance by the Production Code censors.
When Anna Stensgard suggests they have a drink to celebrate Johann's return, her glass changes position between shots. It's first in her hand, then on the table, then back in her hand.
When the German soldiers prepare to burn Andresen's belongings, they pour petrol from British WW2 4-gallon tin-plated cans (flimsies). Pressed steel 'jerry cans' were manufactured in Germany in 1937 and stockpiled in anticipation of war. German forces would have had access to the superior 'Wehrmacht-Einheitskanister' in 1942. Film-makers in 1943 did not.
The machine guns used by the Germans are, in fact, American Browning M1917 and M1919 water-cooled, and M1919 air-cooled machine guns.
One of the fishing vessels is named "Björg". The letter ö is Swedish, the Norwegian version of this letter is ø.
When the Norwegians receive arms from the English, one of them notes that the delivery included "15,000 rounds of .50-caliber ammunition". However, none of the firearms they receive are .50-caliber weapons. The delivery includes British Enfield Mk.III rifles (.303-caliber) and Thompson submachine guns (.45-caliber), but no mention is made of ammunition for these weapons.
When the doctor is treating a wounded villager, Gunnar Brogge is holding a lantern for lighting. The lantern is a U.S. railroad lantern.
The horse-drawn wagons never would have gotten close to the building they were assaulting. Somehow, in this movie, the charging horses are immune to .30 caliber machine gun bullets. Not one was hit in spite of charging directly at them.
The Germans appear to be throwing American or British grenades. But Germany used "potato masher" grenades during both Word Wars. These grenades had a long hollow wooden handle (through which the fuse ran). These handles not only provided protection for their fuses, but assistance for hurdling the grenades further than if there was no handle. No German in this movie threw "potato mashers."
The fact that all the ships are models is quite obvious at the beginning, when the destroyer sails past a few small anchored fishing boats. The "wake" from the model destroyer has an exaggerated effect on the model boats. That would not be as pronounced with real watercraft.
Several Germans appear to be carrying MP-40 submachine guns. But upon closer look, they seem to actually be bastardized Thompson submachine guns, altered by removing their butt-stocks and lengthening their magazines. However, the submachine guns' pistol grips, actions, and barrels reveal them to be, in fact, American Thompsons.
The German helmets are all 1917 as evidenced by the spikes out the side. WW2 had the M37 which would no have been available. The Mauser rifles were all early straight bolt and not the proper K98 as used in ww2.
Near the beginning, Karen (Ann Sheridan) hands Gunnar (Errol Flynn) a map indicating a dashed line from Norway to the northernmost British Isles. Gunnar then states he could find his way to England in the dark, but the dashed line ends in Scotland, a separate country within the United Kingdom.
The German at the start of the movie writes the date in his journal starting with the month, then date, then year (such as "Oct 25th 1942"). This is the American way of writing out dates. But Europeans write the day first, then the month, then the year (the given example should have been written as "25 Oct 1942).
When the shopkeeper decides to shut early, he just walks out the door and walks away without locking up.
At 6:38 the German soldier sitting dead in his chair blinks before he is spun around and pushed to the floor.