Factual error: The strong current of the river the main character falls into carries him to a considerable waterfall. There is not, nor has there ever been, a river of that kind in Flanders' Fields, let alone a waterfall. Anyone who has any insight into the geography of the region will tell you it is flat as can be. The largest body of water, the Yzer, gently meanders and flows into the Channel, even during really rainy times.
Suggested correction: I do not believe that either Ecoust or Croisilles Wood is in Flanders. Both are behind the old German lines at the Somme. That said, there are no bodies of water in that area.
Ecoust and Croissilles are in department Pas-De-Calais in the French Flanders, together with the Belgian flanders they are called Flanders Fields. It is indeed a flat area.
There is a watercourse that goes through Croisilles... But it's basically a ditch.
Continuity mistake: The injury Schofield suffers to his neck is healed by the time that he gets to the front line.
Plot hole: The letter that had to be delivered to the Colonel should have been completely ruined when the Corporal was in the river.
Factual error: The event starts on April 6th, 1917. While the cherry trees are blooming, all the other foliage looks to be in full summer bloom. (00:37:15)
Other mistake: At the start of the trench running scene where the soldiers charge, keep looking at the far left of the screen. You will see a small group of soldiers already running who were not in the trenches. In addition, they seem to be running without weapons.
Factual error: The British medical station is above ground and too close to the front lines. It is well within range of German artillery. Normally, such large open-air stations would be miles behind the front lines. Smaller, underground aid stations would give first aid to the wounded. Then they would move to the larger medical stations.
Factual error: In one scene you can see the bottom of a soldier's boot, and it has a light blue rubber part on it. They didn't make colored soles back during WWI.
Continuity mistake: When Schofield drinks milk, he has no bandage on his hand.
Character mistake: Scene where they push the truck out of the mud; right after they drive away in the background you can see that the other side is flat grass. They never had to do all that.
Factual error: While crossing No Man's Land an abandoned tank is seen. Tanks were not used on this battlefield until April 9th, 3 days later than the date the film is set. (00:20:10)