Other mistake: This takes place in 1914. While the Prussian agents drive away with Hannay, after kidnapping him from police custody, on the road you can see a modern marker for a car parking space, near the corner. (00:25:10)
Other mistake: After David Warner kills a man with his long thin sword, he pulls the sword out and puts it back in its scabbard, but there's no blood on the sword or anywhere.