When Lord and Bruce are at the racecourse Lord's jacket changes to an overcoat with velvet collar.
In the final confession letter, the USSR is referred to as the "United States of Soviet Russia", rather than the more accurate translation of the "Union of Soviet Socialist Republics".
When Frank gives Bob, a Russian speaker, a book as a present, he says that the Russian bookseller recommended it as a reliable translation. However, the book he hands him is a Russian language volume of work(s) by A.S. Pushkin, easily identifiable as such by the Russian Cyrillic letters on the cover, and Pushkin was a Russian poet who wrote in the Russian language. This makes the book a Russian original, not a translation.
About 12 minutes in, Nigel Havers and Michael Caine walk past Cheltenham Boy's College, with Nigel Havers delivering a line that continues, without pause/taking breath, as they walk down Montpellier, which is about 1km away.
Frank Jones had served in the Royal Navy for twelve years, but was wearing a Long Service and Good Conduct Medal; awarded for fifteen years' exemplary service. And the medal being worn was not that of the Royal Navy, but of the British Army.