Factual error: During the battle scene the voiceover says that it is February 1915. The battle being portrayed is obviously the Second Battle of Ypres because it involves Canadian troops and it's the first time that gas was used in the war. The actual battle didn't take place until the end of April 1915.
Factual error: It has already been mentioned that, although the war scenes are set in early 1915, gas was not used until April of that year. There are further inaccuracies regarding this particular scene. The mask worn by the German cavalryman is not the sort of mask that was in use in 1915. It looks like a late WW1 or WW2 gas mask. Furthermore, since horses can also suffer from gas, a cavalry horse taking part in an attack in which gas was used would also be equipped with a gas mask. However, by early 1915, when both sides were dug into trenches, cavalry would not have been used anyway.