Waterloo has several deliberate errors, though the idea that the Prussians wore black may well have been political. The movie was filmed in Russia and used Soviet military units for the 1000's of extras needed. As such the idea of saying the Russians wore black (actually dark blue mostly, grey otherwise for some Landwehr units, etc) identified Blucher and the lads as the same evil bastards that invaded the Motherland in 1941. They hadn't changed a bit.
The most glaring error was when the Royal Scots Greys got skewered by French lancers. Depicted are the single, blue clad Polish squadron of the Guard Lancer Regiment, the remainder wearing the bright red uniform of the previous Imperial Guard Dutch Lancers. IN REALITY, the unit that hit the Scots Greys were Pire's line lancers who wore dark green uniforms with brass Grecian style helmets. But they obviously didn't look as spiffy.
In Kelly's Heroes, the Shermans are real while the Tigers are fabricated shells dropped on the top of Russian T-34s.
Finally a review on Popmatters about the newer Alamo evidently said (according to Wikipedia) "The depiction of Crockett's fate came from memoirs supposedly written by former Mexican officer José Enrique de la Peña, an officer in Santa Anna's army who fought in the battle. It was the first film to show Crockett executed as a prisoner of war; all others had depicted his death as occurring during the battle."
Ciao, the Author