Revealing mistake: The French tanks that attack Paul et al. Towards the end of the second act are supposed to be Saint Chamond tanks. There is only one of those left in the world, and it is not on screen: the undercarriage and tracks don't even come close to looking like that of a Saint Chamond, because it's actually a BMP-1 dressed up to look like a Saint Chamond.
Continuity mistake: Near the end we see a poster of a woman being pinned up in the German trench. This poster is folded and rolled at the bottom. At the end as the camera is panning out, the poster is flat and not folded.