Star Trek: 12 Biggest Ever Retcons
3. Kirk Has A Nephew
Operation: Annihilate marked the first time Kirk's family was introduced on the show. His older brother Sam appeared briefly (albeit already dead from an alien parasite), and his sister-in-law Aurelan and nephew Peter feature more prominently in the episode. After Aurelan dies from the same parasite that killed Sam, Kirk is the only family Peter has left.
Kirk having an orphaned nephew seems like a pretty important development, but after this episode, Peter is never referenced again. What happened to him? It seems unlikely that he would have stayed on the Enterprise, but to never hear from him again seems like a pretty big oversight on the part of the writers. And speaking of disappearing family members...