Doctor Who Series 9: 22 Easter Eggs You Need To See

7. The Zygon Invasion

The UNIT Dating Controversy is a continuity issue in Who (one of many, but this one has its own name and even a documentary about it) that dates back to the 70s and 80s. Although fairly layered, the crux of it is that Mawdryn Undead says the Brigadier retired from UNIT in 1976. However, the events of The Invasion, the first story in which UNIT properly appears, approximately occurred in 1979. Thus fans have been left baffled as to how he could've retired from something that didn't exist. In The Zygon Invasion, Kate Stewart says that the last time the Zygons invaded was in the "70s or 80s." Given that Kate is the daughter of the Brigadier, it's likely this was very much a knowing reference to the Dating Controversy. It wouldn't be the first time this has happened on screen, with David Tennant's Doctor making a remark about it in Series 4's The Sontaran Stratagem, stating he worked with UNIT in the 70s or "was it the 80s?"