When many, many people were stubbornly insistent that the mole in Hue city is dead, and then it turned out to be someone that could in no way of survived the Vorkuta escape, I was suspicious. It was obvious that there was a twist, and if I know that, I have to try and pick it apart; predict it. So I can't say I was that surprised.
When I started to think that Reznov post-Vorkuta was a figment of your imagination, I started paying attention to his physical actions. Other than various weapons, he doesn't interact with anything. And no-one ever shoots at him during the many firefights. So I kind of worked it out ahead of time. Go me, ruining the story for myself...