Retroactive continuity or retcon is the adding of new information to "historical" material, or deliberately changing previously established facts in a work of serial fiction. The change itself is referred to as a "retcon", and the act of writing and publishing a retcon is called "retconning". Retconning can be done either on-purpose, or accidentally, wherein a break in continuity is not noticed until later and is then 'blessed' by later writers or editors.
Retcons are common in comic books, especially those of large publishing houses such as Marvel Comics and DC Comics, because of the lengthy history of many series and the number of independent authors contributing to their development; this is the context in which the term was coined. Retconning also occurs in soap operas, movie sequels, professional wrestling, video games, radio series, series of novels, and can be done in any other type of episodic fiction. It is also used in roleplaying, when the game master feels it is needed to maintain consistency in the story or to fix significant mistakes that were missed during play.
It seems someone knows his Star Trek series shows very well.
Chekov was never in "Space Seed," which was the episode where Ricardo Montalban made his grand appearance as Khan.
I remember seeing an interview with Chekov about that fact, he told the interviewer and guests that he was stuck on the john after eating some bad stew with a bad case of well, you know, and Khan was banging on the restroom door cause he had to take a dump too.