Continuity mistake: The officer is driven quite mad by the psychic attack of the demon and shoots one of his own men in the head. We see the soldier fall to the ground clean, yet in the next shot a lot of brains (poked by Andy Serkis's character, with his pistol) and blood suddenly appear.
Deliberate mistake: The entire time they spend in the trench, before they attack, Quinn has a white sheepskin jacket. It rains throughout the film yet with all the mud about you would think in about a day that this jacket would turn dark brown. It is still fairly white even when Quinn dies.
Factual error: Captain Jennings is referred to as the CO (Commanding Officer). In the British Army only battalion and regimental commanders are referred to as COs. Company commanders like Jennings are OCs (Officers Commanding). Not a mistake any British soldier is likely to make.