When Natasha is sitting next the dying Prince Andrei's bed, she leans her both hands on her legs. In the next shot, when Kolya enters in the room, her right hand is on the top of the bed pole.
In the first battle, Prince Andrei dismounts to take the flag of the shot soldier who is holding it. Then the wounded soldier kneels twice.
After the opera, when Anatole and Natasha meet, he approaches her from behind and puts his left hand on her shoulder. The next shot shows them a little way from each other.
When Natasha and Andre first go onto the dance floor, Natasha is holding only a program, which she shifts from her right hand to her left just as they begin to dance. After a camera change to others getting up to dance, the focus returns to the couple. Natasha is now holding her train with her left hand (which the dress didn't have at the beginning), and the program is gone.
When Natasha is waltzing with Prince Andrei at the ball, Andrei and some of the other men on the dance floor can be seen wearing swords, which swing around as they move. Exactly for that reason -- they would have whacked into other people -- swords were not worn while dancing.
Just after Andrei dies, while Natasha is at his bedside, he swallows and his Adam's apple visibly moves.
The marching band in the opening parade are all playing modern musical instruments.
When Sonya enters Natasha's bedroom where Natasha has retired after meeting Anatole at the opera, she appears to turn on an electric lamp! In 1812, candles and fireplaces were the only source of lighting. Kerosene lamps did not appear until mid 19th century.
During the duel scene, the participants are using percussion pistols which were introduced circa 1820's, fifteen years after the events of this film.
Although the Battle of Austerlitz was fought in December, the trees seem to be in full summer bloom.
At the 1 hour 55 minute mark, as Henry Fonda walks down a crowded street, the dolly track is completely visible for several frames.
Napoleon refers to Moscow as the capital of Russia. Moscow had not been Russia's capital since 1732. Saint Petersburg was the capital during the Napoleonic wars. Moscow was not restored as Russia's capital until 1918.
Napoleon is depicted much shorter than other people, which was not true. Actually, he was average size. At 1.57 metres (5 ft 2 in), he had the height of an average French male but was short for an aristocrat or officer. On the other hand, Napoleon surrounded himself with tall bodyguards and was affectionately nicknamed le petit caporal (the little corporal), but it reflected his reported camaraderie with his soldiers rather than his height.
All Russian and French characters speak modern English all the time. Only priest's chants and song singed by the people on the raft are not in English. Most texts are also written in English, such as letter. The opera's hand program is in Russian.