When Guus goes to Holland for the first time to instruct the resistance group, he is walking in the street with his transmitter in a trunk and in the street he passes a window which displays a poster which says "staking brengt alleen ellende over uw eigen volk". This instruction (which Guus gives) in the movie takes place in 1942, but this poster wasn't issued by the Germans before the winter 1944-1945 as it refers to the great train strike in the winter of 44-45.
After the first failed attempt to flee to England where John is captured, Erik meets with Guus and asks Guus to hide him. In the background, Dutch fascists are selling newspapers and shouting, "Russian Front Collapses." Later John is executed and a subtitle tells us the date is April 10, 1941. Germany did not invade Russia until June, 1941 so there was no "Russian Front" before then.
Erik's eyesight is so poor that he is unable to hit targets immediately in front of him with an automatic weapon, yet shortly afterward, while being interviewed by the queen, he is able to spot the lovemaking couple through the small window at a much, much greater distance.