Factual error: George Smiley stays in a hotel near Liverpool St Station, London to investigate the spy at the heart of the circus. There is one scene where we see, from above, a train leaving the station. Liverpool St station is electrified using overhead wires - the train is leaving a station using a different electrification system called 3rd rail which has never existed at Liverpool St.
Factual error: Control seems to have left behind some turntable package material for a deck called Dual CS 505-1. This model, however, only went to production in 1981.
Visible crew/equipment: In the scene on the runway a sound man with a boom mike is reflected in Smiley's glasses.
Continuity mistake: Bill Hadon is on a call with the Hungarian embassy. He's holding his phone in his right hand talking on it, but then slams it down with his left hand.