When we first see Number 2 in his headquarter chamber, his pocket handkerchief is a One Point Pocket Square Fold. Then the next scene in the hospital it's a Flat Pocket Square Fold.
The road in front of the house that Number 6 used to live in changes traffic direction. In Many Happy Returns traffic ran from the left to the right. In Do Not Forsake Me, Oh My Darling, it runs the opposite direction.
Number Six, in the Colonel's body, returns to his house after meeting Sir Charles Portland. He pulls up level with his front door, and the hearse with license plate TLH 858 (seen in the opening sequence of every episode) is parked a short distance behind. Then in a brief wide shot, Number Six's car is parked beyond the front door and level with the red mailbox, while the hearse is missing. In the next shot the hearse reappears.
When the Colonel first enters Number Two's room, the doors behind him close twice.
Just after Number Six puts away an atlas when he finds Kandersfeld, there's an odd jump cut and the projector and a wire suddenly move.
In several wide shots when Number Six (in the Colonel's body) is driving his Lotus Seven, the driver is clearly Patrick McGoohan rather than Nigel Stock. This is because the footage is taken from the opening sequence.
When Number 2 and the Colonel are discussing the assignment in the control room, in the wide shots there is an obvious stand-in for the Colonel.
The money #6 gets out of his hiding place of his London flat is US currency. $20.00 bills are seen.
When Number Six fights with Potter, one stunt double wears a heavy grey wig that does not match either actor.
In the headquarter chamber of Number 2 we frequently see a tea service on a cart. The fact is there is a ramp inside the chamber, but outside the door there are 3 steps. Therefore this would be very inconvenient to always bring in the tea service in this manner.
Number Six's mouth doesn't move when he tells a waiter that he took some photographs.
In the early scene where Nigel Stock as the 'new' Number Six in a POV shot spots himself in the mirror, the reflections of the camera, operator and another man can be seen in the mirror.