As far as I am concerned, the most egregious mistake in 'Legacy' is the appearance of the legendary George Smiley at the end of the book. How old is Smiley?
The first chapter in the very first book of Le Carré ('Call for the dead') is entitled 'A brief history of George Smiley'. Here we read Some time in the twenties when Smiley had emerged from his unimpressive school and lumbered blinking into the murky cloisters of his unimpressive Oxford College ... On a sweet July morning in 1928, a puzzled and rather pink Smiley had sat before an interviewing board of the Overseas Committee for Academic Research. So: Smiley completed his degree in 1928, meaning that he was born around 1907.
'The honourable schoolboy', which is the second part of the retro-styled 'Karla trilogy' states that ... the true genesis was Haydon’s unmasking by George Smiley and Smiley’s consequent appointment as caretaker chief of the betrayed service, which occurred in the late November of 1973. This gives the date for 'Tinker, tailor', making Smiley about 67 years old at the time. This is reasonable.
'The spy who came in from the cold' has to take place after the Berlin Wall was erected in late 1961; 'Legacy' contains an account of the 'trial' which takes place in November 1962. 'Legacy' itself starts as a verbal account by Smiley's sidekick, Peter Guillam, who says on the opening page "What matters to him [a professional intelligence officer] is the extent to which he is able to suppress them [human feelings], whether in real time, or in my case, fifty years on" [emphasis mine]. This dates 'Legacy' to about 2011 ... at which time George Smiley would be 104 years old!!! Even dropping ten years off his age in the reboot 'Tinker, tailor' would make him 94 years old ... very unlikely to be still researching in Swiss libraries.
Narrator Guillam's timeline is also problematic: 'Legacy' has him aged 8 at the end of World War 2, which means that he was born around 1937. 'Call for the dead' takes place in 1961, and Guillam is duty officer on the first night of the story - at the tender age of 24. This doesn't leave much time for his training and active service abroad. Also, it seems unlikely that he would have been advanced at such a tender age to the position of trust which he occupies in the historic parts of 'Legacy'. On the other hand, the present time of 'Legacy' would have him aged about 75, which would make him slightly too old for the physical feats which he considers. Some of these problems could have been alleviated by setting the present day parts of 'Legacy' in 1995 or even 2000.
There is something else in the book whose significance escaped me the first time I read it - the episode in which Guillam befriends Liz Gold before she starts working in the library where she will meet Alex Leamas. I'm not sure exactly what the point of this is: it might be that the Circus was setting up Gold to meet Leamas - but elsewhere it states that Gold had been working in the library for several months before Leamas turned up, which implies that the planning had been in motion for quite some time. This part simply does not ring true and seems an unnecessary embroidery. I will have to read it again - when I am not running a fever - to see whether there is something that I have missed.
Basically, what I am saying about this book is that apart from its enjoyment factor, casting light on the background of an operation (this part is truly fascinating), author Le Carré seems not to have done his homework, and in the attempt to add interest has added things which are demonstrably incorrect. Presumably not every one reads the books with such an analytical mind.
Correcting something which I wrote before, according to 'The spy who came in from the cold', Karl Riemeck worked in the secretariat of the Praesidium of the East German Communist Party, and was codenamed Mayfair. This jibes with what is written in 'Legacy'.
I am not the only person who has commented on George Smiley's age. Author Le Carré says in a recent interview "he’s [Smiley] said all he has to say. Also, he’s about 120".