The 17th Lancers accompanying Lord Chelmsford and his HQ very prominently during the movie were actually sent to South Africa only after the disaster the movie depicts, together with the 1st Dragoons. At the early stage of the war the movie shows there were only some local and native mounted units at hand, and some British infantry that were mounted for the particular campaign.
Burt Lancaster plays Col. Anthony Durnford with a strong Irish accent. In reality, Durnford came from an Anglo-Irish family from the Ascendancy and would not have spoken with such an accent, he would have sounded like an upper class Englishman.
As Colonel Pulleine writes his last letter in his tent, he is holding the pen in his left hand. The shot switches to a view over his shoulder, and the pen is now in his right hand. Then as a Zulu bursts into the tent, Pulleine drops the pen from his left hand again to grab a revolver.
After talking to the Quartermaster, the bugler sounds the last post. While today this is used in memory of fallen soldiers, back then it was literally the signal that the last post had been checked ie that all the lookouts were awake. The bugler wasn't at a lookout post so shouldn't have sounded that signal.
As the column is crossing the river we see two native bearers carrying an ammunition box, which should be quite heavy. They stumble in the current, dropping the box, which then bounces to the surface and starts to float off. The ammunition box should have sunk.
Bod Hoskins' character is addressed throughout the film Colour Sergeant Williams - and wears the insignia of that rank. However, in the credits he is referred to as CSM (Company Sergeant Major) Williams.
In several shots (most notable when the Zulus first come into view of the camp) it's obvious that many of the Zulu warriors are carrying fake shields: Instead of cowhide, the shields are obviously just painted on some sort of a plate, probably plywood.