Factual error: The rifles the British soldiers carried in the film were Lee Enfield rifle No. 4's, these did not go into production until WW2. They should have been using an earlier version of the Lee Enfield rifle.
Continuity mistake: The soldiers' breath is seen in some scenes, but not others.
Factual error: There is, and never has been, for that matter, a Private First Class rank in the British army.
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.
Plot hole: Towards the end of the film when Private Shakespeare is talking to the wounded soldier, his coat and hair are suddenly dry, strange when the majority of the film is set outdoors in the rain.
Factual error: After the opening attack, when the survivors start trying to find their way back to safety, the commander orders one of his soldiers to "take point." Take point only came into common use during World War 2, among American forces, so would have been unlikely to be used by British soldiers in 1917, though it had been used, to a lesser degree, mostly by Americans prior to this.
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.
Factual error: Tate refers to McNess as a "plank". The first recorded use of "plank" to mean "idiot" wasn't until 1981.
Factual error: Steel helmets were introduced into the British Army in 1916. After their introduction no soldier would have gone into the front line, and certainly not over the top, in a peaked cap, as several of the soldiers do (the film is set in 1917).
Factual error: One of the soldiers says that it isn't a holiday camp. Holiday camps were not introduced until the 1930s, over a decade after the film was set.
