At the Start of the film when steed drops the box of macaroons on the desk in the first shot the bakery name is at the front and the box is square on, the following cut the box has turned the text on the side of box is now at the front and the box is slightly angled
During the scene in the boardroom with the teddy bears, De Wynter says that anyone who wishes to leave can do so, and a payment of one million dollars will await them. However, he clearly mouths the word "pounds" instead of "dollars."
When Emma Peel plays the piano, none of the dampers inside the piano moves up and down. Even though we discover it's a player piano, the dampers would still move every time a key is depressed. The only time the keys are in sync with the sound is in the final closeup as the Chopin piece ends.
When Steed and Peel are being chased by Bailey's flying insects, we see Bailey's control panel, with red lights that correspond to the number of insects flying. The screen shows fewer and fewer lights as the bugs crash. Only 2 insects are left at the end, but the control panel shows more than 2 red lights.
Between the two bursts of full auto fire from Alice's Thompson sub-machine gun, the bolt is seen to be closed. Thompson sub-machine guns are all fired from an OPEN bolt position (in order to cool the chamber and prevent rounds from "cooking off").