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.
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.