Factual error: Christopher Robin and his schoolmates undergo the army medical (which he fails) at the outbreak of war. In fact, Christopher Robin had left school and was at Cambridge by the time the war broke out. In addition, those that pass the medical go off to war in privates' uniforms. As the school (Stowe) is a prestigious public school, almost all would have been commissioned and never served as privates.
Factual error: The musicians in the (real) Band of the Coldstream Guards that plays Happy Birthday for Christopher Robin are wearing their 21st-century uniforms and medals.
