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.
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.
When Ricki Tarr makes a call from a red British phone box he puts the money in before dialing. This is incorrect - British public telephones at the time required you to wait for the call to be connected (indicated by a rapid beep-beep-beep tone) before inserting coins (and then periodic 'feeding').
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 at the docks Irina and Ricki drives past a light blue ship with a cream colored superstructure. This can only be a MAERSK LINE container vessel. We see a bit of a crane, mounted on the starboard side, which defines a class of container ships (of which the ARIZONA is one). This series of ships were built in the 1990's.
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.
The disappearing green towel: At the beginning of the movie - as the credits are still rolling, Smiley (Gary Oldman) after having completed a swim in a "pond," thereafter in street clothes leaves the optometrist's office with his bulky swimming "towel" under his arm. The very next scene he is walking through a snow covered park hands in pockets but without the towel. Then the next two scenes the towel is back again - this as he first walks along a street and then arrives at his front door.
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.
When Smiley and Esterhase meet on the runway, the plane that stops behind them has the left prop blade to one side. When moments later Peter is shown opening the door, the same prop is now straight up.
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.
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.
Though David Dencik lets his native Swedish accent slip when Toby Esterhase says the name "Polyakov" - he says "Poliakoff" - during his confrontation with Smiley, it's not considered as an error since Esterhase isn't a native British. As established earlier in the film, he was rescued by Control in some other European nation (the quote in fact was "I should have left you where I find you") and Toby probably wasn't able to fully master the English language with fluency.
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.
SPOILER; When Prideau executes Haydon by shooting him in the face, Haydon is standing close to the fence and collapses slowly slightly to his right. When the camera shows his prone body, he is stretched full length, face down, perpendicular to the fence with his head toward the fence. Based on physics this is impossible.
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.
When Smiley meets with Esterhase at the airfield there is a lot of heat haze in the background and the trees are in full leaf, suggesting a hot day in summer, but all the characters present are in overcoats. Also there is enough wind in the foreground to make their coats flap about but the trees in the background do not move at all.
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.