Star Trek: 12 Biggest Ever Retcons
2. The Number Of Kirk's Nephews
Operation: Annihilate was the first time Kirk's family was introduced in the series, but he first referred to them in What Are Little Girls Made Of? When testing the memories of a duplicate version of himself made by Dr. Corby, he mentions his brother Sam. The duplicate supplies that Sam's full name is George Samuel Kirk and also mentions that Sam has three sons. This would imply that Peter Kirk, Sam and Aurelan's son in Operation: Annihilate, has two brothers.
However, there is no indication that Peter has any siblings in Operation: Annihilate. Everything about that episode suggests that Peter is an only child. So Kirk goes from having three nephews to just having one to seemingly not having any nephews at all.