The first 10 minutes of S1E2 is filmed at what is now the Pearse Museum in Dublin.
The British officers are seen with rank insignia on their shoulders. While some officers did this in the trenches in 1916 it was not adopted officially and outside of France until 1917.
In the beginning of the first episode of the second season the US-American delegation arrives in a vehicle flying the modern day stars and strips. In 1920, however, the Old Glory Flag had only 48 stars.
Two members of the Game of Thrones cast are in the series, Michelle Fairley (as Catelyn Stark) and Ian McElhinney (as Barristan Selmy).