In one of the flashbacks to the Christmas party, presumably before Control resigns in 1973, a "Lenin Santa" has everyone sing along to a recording of the Soviet National Anthem. The version that is played is the 1977 version - the original 1944 words fell out of favor after Stalin's death in 1953 and the anthem was played without words until the new version in 1977.
Julio Iglesias' cover of the song 'La Mer' used in the movie's final scene is from a live album recorded in 1976 ('En Directo Olympia'); it wouldn't have been on the turntable at the last Christmas party which had Control and Prideaux in attendance, as that was presumably in 1972.
When George Smiley visits Connie Sachs, she brings in a tray with tea and milk in a bottle. The milk bottle is a modern, thin-walled, short-necked model. These were not introduced in Britain until the 1980s. It should be an old thick-walled, tapered version.
During a roof shot going down the building to the road in Istanbul, 2 split type air conditioners can be clearly seen mounted on a building wall. Split air conditioners came to the Turkish market in mid'90s.
When Smiley is first shown waiting on phone in the secret flat, you can see his shoes removed and under table. In the next scene a few seconds later, he is shown again with shoes on, which he then proceeds to remove.
Lacon meets with Alleline and Bland. He is holding a sheet of paper prominently in his right hand. As the camera angle changes to Alleline, who says "Operation Witchcraft needs to remain secret," Lacon is instantly holding a cup of tea in his right hand instead.
The hotel where Smiley carries out his research supposedly overlooks Liverpool Street Station. The tracks seen from his window have 3rd rail electrification, whereas Liverpool Street uses overhead live wires.
The price of the train ticket to Norwich was quoted as 'One pound fifteen'. By 1974, this price should have been decimal, but appears too cheap for a ticket for that distance during a year of high inflation. £1. 15 shillings (1.75) would have been allowed in 1973 (just - as we introduced decimal in 1971) but not 1974.
Despite Control being so secretive that not even his own employees know his name, Connie Sachs is able to show Smiley a picture of him a military uniform. With that picture Connie (or anyone else in the Circus records) would have been able to identify rank, regiment, and date - and therefore with a little research - Control's name.
Connie Sachs shows Smiley some old photographs. Underneath the photographs is a postcard. A postmark is clearly visible on the stamp but it does not run through onto the card itself. This is probably because the film makers used an old postmarked stamp on a new postcard.
The goof item below may give away important plot points.
When Smiley and Guillam are going through Control's flat after his death - which occurs in 1973 (based on the last document he signs before he leaves The Circus, of which there's a clear close-up showing the date) - there's a box for a Dual CS505-1 turntable - which wasn't made until 1980.