Number Six walks past the same jukebox twice. It is easily identifiable by the Lesley Gore record in it.
A very obvious fake beard attached to the 'dead' Number Two's chin is clearly visible for a couple of seconds as he is being carried away. This was because Leo McKern had inadvertently shaved off his real beard prior to filming.
The door to the council chamber is marked WELLCOME.
The shadow of the camera is seen when there's close-ups of the various Earth globes.