Revealing mistake: The cannons shown are firing blank charges, which produce no recoil. Recoil has been simulated by towing the guns backwards when they fire. The problem is, there is always a distinct delay between the discharge and the "recoil." The cannon also seem to largely fire explosive shells. At no point do we actually see a cannon ball rolling or bouncing along the ground or into any group of soldiers.
Factual error: Throughout the film the Allied troops (with the exception of the Artillery Corps) wear scarlet and the French wear blue. However, combatants in the battle wore much more diverse uniforms. Wellington, for example, had Dutch, Belgians and Germans (the King's German Legion) under his command who had different coloured uniforms - not to mention one of his most famous regiments, the 95th Rifles, who wore their traditional green. None of these are shown in the film at all.
Factual error: Towards the end of the film, the British are shown to be re-capturing La Haye Sainte. The British never re-captured it until the battle was over.
Factual error: Toward the end of the movie, when the Prussians finally arrive, they enter the scene from the left of the frame; this would have been the British/Allied right flank and the French left flank since the scene is shot from the French point-of-view. In actual fact, the Prussians entered the battle on the British/Allied left flank and the French right flank, so they should have entered the scene from the right side of the frame.
Factual error: Before the battle it rains. The dye used in the British red uniforms of that period was so poor, that any rain would make it run. As a result, most of the British army went into the battlefield with "pink" trousers, rather than the unstained trousers shown in the film.
Factual error: When Ney comes to make his report to Napoleon after Quatre-bras(Where Ney engaged the Allied army), he throws a captured British flag down before Napoleon. This flag is a green Regimental flag, with a sphinx insignia - But the only British flag captured at Quatre-bras was the Kings Colors of the 69th, which would be a great Union Flag with '69' in gold leaves.
Factual error: The British army is wearing "normal" (left/right) boots. Wellington's soldiers were issued with two identical boots that could be worn on either foot, and were actually instructed to swap them from foot to foot regularly to even out wear and make them last longer.
Revealing mistake: During the battle, both the Allied and French commands spot unknown troops marching towards the battlefield. They are marching through what looks like a wheatfield, following paths which have already been made. However, these new troops are marching on a route so far unused by either the French or Allied armies, so therefore the field should have been untouched - these paths were made on the first take.
Audio problem: When Ponsonby is being pursued by the Polish Lancers across the muddy field, he gives his watch and snuff box to the young bugler and tells him to ride on and save himself. However, his lips do not correspond to the words.
Factual error: In the opening text, the movie states that England was one of the countries that defeated Napoleon at Leipzig. However, England did not participate in that battle; Wellington had just defeated Soult and launched an invasion of southern France at the time. Interestingly, Sweden did send about 30,000 soldiers to fight at Leipzig, but is not mentioned.
Factual error: At no time during any of the battle scenes do we see any of the soldiers actually loading their muskets, despite the lengthy and time-consuming process which it took to carry out the priming and firing of these weapons.