Star Trek: 10 Times Canon Got In The Way
2. Who Are Ya?
If ever there's a prize for best reason for allowing canon to slip by unnoticed, it has to go to Walter Koenig.
With the full knowledge that Chekov had never met Khan on the Enterprise in Space Seed, Koenig chose to keep stump in order to keep his role as sizeable as it was in the movie sequel. In fact, even director Nicholas Meyer had clocked the issue and likewise decided it was best just to ignore such a (trivial) detail.
Khan never forgets a face so if viewers have to make up some form of head canon, Chekov must have been hiding away on the Lower Decks (no, not those Lower Decks or that Lower Decks from TNG) of the USS Enterprise just waiting to take up that bridge position for season two.
In retrospect this would have worked with any of the other crew had they chosen to swap out Chekov for Sulu for example but then it's impossible not to think that only Walter Koenig could manage such impressive screaming when the ceti eel finds a new home or carry off the line "Dey put creatures in our eeeears..." with such aplomb. Thus the first canon, in timeline, sequence error was born and a legend created!