When Brian Donlevy stops in alley to change his hat so not to be recognized. After changing his hat, the next shot (only a second later) shows that he also not wearing his turtle-neck sweater, but instead wearing a shirt and tie.
After the German soldier shoots open the study door at Czaka's house, the door is opened yet there are no bullet holes, around the door handle or in the door or frame, even though there were at least 10 rounds fired by a machine gun.
The pimple on Gestapo Chief Haas' face keeps moving around---in the opening scene it's above his left eye, later it's on his right chin, and near the end when he's looking at himself in the mirror as he squeezes it, the close-up shows it on his left cheek.
As the boy (Boda) picks the lock to open the door where Jan is tied up, he dumps the contents of his pockets on to the floor outside the room. Once he gets the door open he immediately goes inside and releases Jan. Once Jan is released, he and the boy rush out of the room but the things are no longer on the floor.
In reality, Heydrich was assassinated by a team of Czech exiles sent back to the country by the British government.
Heydrich was not shot. The gun provided to the assassins jammed. One of the men threw a bomb at the car that landed near the wheel and exploded, sending shrapnel through the seat and Heydrich's clothes before injuring his spleen, diaphragm & lung. He died from sepsis in the hospital days later.
The character of Reinhard Heydrich wears the collar tabs of an SS-Gruppenfuehrer (group leader, roughly lieutenant general), three oak leaves. As Reichsprotektor of Bohemia and Moravia his rank was higher, SS-Obergruppenfuehrer (higher group leader, roughly full general). The proper insignia for this rank in 1942 was three oak leaves with a single pip underneath.
When the German police disperse the crowd who've surrounded Ms. Novotny after the cab drove off, some of them start beating a young boy with wooden clubs. The clubs can be seen bending as they wield them.
During their knock-down-drag out fight, Gestapo Insp. Alois Gruber kicks Jan Horak hard to his left cheek and hard to his ribs; then knocks Horak to the floor with a powerful left hook, and hits Horak hard on the back his head with a heavy piece of firewood, knocking Horak unconscious. Gruber ties Horak to a bed by binding his wrists and ankles. A day or two (?) later, Horak, with no visible sign of any injury, smothers Gruber to death by pressing a pillow over Gruber's face for less than 10 seconds. Thereafter, Horak is interrogated in a small office where he says he's been to a doctor and obviously tries to conceal his (presumably bruised) left wrist. None of the three highly trained and experienced Gestapo interrogators take any notice of Horak's clumsy attempt to conceal his wrist from their view or inquire about his visit to a doctor.
When Ms. Novotny is being driven in the hansom cab to the Gestapo, there is a shot of the traffic in Prague with people driving on the left. When the Nazis took over in '39, they ordered everyone to drive on the right (as they did in Austria the year before).
When Gruber leaves an SS man behind at Svododa's apartment building, he points at the apartment and tells him "if those three go anywhere;" but they had just searched the apartment and had only found two people there.