During the fight in the lake, Mrs. Peel's hair goes from wet to dry and then back to wet.
When the 'murderer' turns up at Unwins house, his car is visible at the side of the house. When Steed arrives a few seconds later, the car has disappeared.
Unwin tries to open the front door of his house and finds it locked. A minute later, Skelton opens the same door easily.
When Rathbone is driven off after collecting the suitcase, the car starts off closer to the steps than it was when it parked up.
The folding of Unwins handkerchief changes during one of the 'murder' scenes at his house.
While in the car outside the bank, Emma Peel has a radio conversation with Steed, who is inside. During the whole conversation, she is holding the transmit button down, and would never have heard any of Steed's comments. Later on, Steed does the same thing as he prepares to follow George Unwin.
When Steed is following Unwin, the location transponder sound effect is actually a naval warship sonar transmission. In fact, the echo is clearly audible and indicates a target that has a range of about 500 yards and is moving rapidly towards the attacking warship. This is discernible by the higher pitch, or doppler, of the echo.
The boxes of halfpennies that Steed stacks up are empty as can be seen by how light they are to carry and how they move when stacked.
When Emma drives off after observing Rathbone collecting the suitcase, the rear view of her driving off shows that it is a different actor in the driving seat as her hair is too short to be Diana Rigg.
As Mrs. Peel follows the baddie back to his leader, the camera dolly is visible in the lower corner of the screen.