Factual error: Throughout the film the British infantry are armed with carbines. These were intended for issue to cavalry and are much shorter than the infantry issue Martini-Henry rifle.
Other mistake: When Melville and Coghill climb out of the river with the colours you can clearly see that the flag is drenched. a few minutes later when they are trying to hold the Zulus at bay you can see the flag fluttering in the wind. It's impossible that the material could dry that quickly when it's that wet.
Other mistake: The scene where the British army is crossing the Buffalo River is in the historic place but they are crossing in the wrong direction.