A short scene with Finch buying a newspaper on Piccadilly Circus is footage taken from Der Zinker (1963). Although wearing a bowler hat, Arent's sideburns are visibly shorter than in preceding and following scenes.
Maurice Messer threatens his companion with a sword cane. He presses the blade against the chest of the man. Cut to a different angle and the blade is much higher, almost at the shoulder of the man. Cut back and the blade is back at the chest again.
A view into a yard trough a window in Arthur Milton's house is obviously shot through a window set on the top of a structure. A shot from the inner yard reveals that there is no house where the room should be. In the "room" sunlight can be seen reflected on the actors which could only be possible if the room had no ceiling. Furthermore, a shadow of the camera can be seen on actor Siegfried Lowitz.
When Sir John and Inspector Higgins are at "London Airport", they are standing in front of a City Map. The Map is actually one of Hamburg, Germany, where parts of the movie where shot.
Towards the very end, in one of the last scenes, the microphone boom can be seen in the top right corner of the picture. It's very dark and it is there for quite some time.