Just before the Armistice is signed (historically, November 11, 1918) but after the Kaiser's abdication and the fall of the German government, Jack tells a group of German officers that he will see them conveyed to the Eastern Front. To the degree an "Eastern front" existed in World War I, it ended with the Russian surrender in 1917.
When Anne and Gilbert tell Diana and Fred they can stay at Green Gables, the gables are clearly no longer green, they are brown. However, in the next scene, when looking at Green Gables over the valley, the gables are clearly green again.
When Anne and Gil reunite, we see that Gil is a lot older than the last time we saw him. His hair is much more gray and his face has aged. However, in the next scene he is no longer aged and his hair is no longer gray. This can be explained in the commentary as the director and producer state that they shot the scene several months prior to the scene before.
(at around 54 mins) When Gil is discussing whether or not the pregnant lady should be admitted, Dr. Powell grabs Gil's arm to further discuss the schedule. When it goes back to that shot, Dr. Powell is holding Gil's jacket.
When Anne, Gilbert and Jack are riding home on the train, and Anne is talking to Jack, he is smoking a cigarette, which he throws on the floor. After stamping it out with his shoe, it sticks to the sole of his shoe, and can be seen smoking heavily as he props his foot on his other knee. He actually shakes his foot to get rid of it! You can almost see Megan Follows trying not to laugh. Then he puts his foot down and stamps it out completely.
When Anne and Elsie are trapped in the train after being captured by the French, Elsie says that they will be killed at dawn. However, as we see from the amount of light around them it is way past dawn.
When Anne is in the trench with Jack, Collette and the baby, she tries to take the horse to go back to the field hospital. When she falls back into the trench due to an explosion, the 'dead' soldier she ends up next to blinks.