Star Trek: 12 Biggest Ever Retcons
9. Spock's Human Ancestry Was Much Farther Back
In Where No Man Has Gone Before, Spock makes a comment to Kirk during a chess game about one of his ancestors marrying a human woman. Kirk then jokes about how tough it must be having "bad blood" in the family like that. This would suggest that Spock was not directly half human but that his human ancestry was originally much farther back in his family tree.
Later episodes would establish Spock's mother as Amanda Grayson, a human woman who married the Vulcan abassador Sarek. This changed Spock's background from a random human woman marrying a Vulcan ancestor several generations back to the the more immediate half human and half Vulcan.