In the London hotel room as Barney and Willy prepare to swap the real diamond for the fake, Barney unscrews a US style power socket from the wall, totally unlike ones used in the UK even in the 1960s.
When Barney is getting ready to switch the diamonds he removes the electrical outlet from the wall. There is no electrical box behind the outlet so there is nothing for the outlet to be mounted to. The outlets on both walls seem to be free floating and not anchored anywhere.
The Cullinan diamond, the largest found in real life was only 3106 carats and is valued at $400 million. A diamond of 27,000 carats should therefore be valued at about $36 trillion in 2017 terms and probably not much less in 1969 terms.
The electrical sockets Barney removed to retrieve the diamond are wired incorrectly. The black and white wires are connected to one side of the receptacle. This would result in a short circuit to the incoming side and no power getting to other outlets down the line.
Barney (Greg Morris) has to work close to the wall near the opened electrical outlet to maneuver the "claw" mechanism when he and Willy (Peter Lupus) switch the diamonds in the adjacent hotel room. Because of this, a large smear develops on the wall next to the open electrical outlet where his head rubbed against the wall revealing that super secret agent Barney wears makeup.
The oft proclaimed "perfect diamond" has occlusions visible under simple magnification (possibly "bubbles" in the glass or plastic prop since they are at least consistent and so most likely not dust)
Since both men in the hotel room were unconscious from the knockout gas, all Barney had to do was quickly unlock the door and enter the room to exchange the fake diamond for the real one instead of the elaborate and time consuming scenario using the hook through the wall.
As Barney tries to put the playful cat to sleep with the knockout gas, he is close enough to the nozzle that he should have put himself to sleep as well.
When the Police Inspector enters the room to return the "stolen" diamonds to Cinnamon, the edge of the entrance door has no strike plate or mortise lock to hold it in place when closed.
As Barney is maneuvering the mirror with the 'reacher claw' you can see a quick glimpse of the studios large square lights reflected in the mirror.
As Cinnamon and Prime Minister Durvard travel in the Rolls Royce, supposedly through the streets of London, they are driving on the right side of the road. In England, they drive on the left. The road markings are noticeably not found on UK roads and, as they pass through a junction, there's a bright red 'Don't Walk' US pedestrian crossing sign clearly visible.
Van Meer says the huge diamond is about 27,000 carats and would weigh "seven or eight pounds". 27,000 carats is actually about 12 pounds.