10 Biggest Doctor Who Retcons
7. The Reapers
The fantastic Ninth Doctor story Father's Day introduced us to fearsome bat-like creatures known as The Reapers. The Doctor tells us that these winged monstrosities are summoned every time a paradox is created, to seal up the wound and put time back on track. In this case, they appeared as a result of Rose going back to the past to prevent her father's death.
Despite the Doctor's insistence that the Reapers are an ever-present threat , Father's Day is the only time they appear. As soon as the episode is over, they never materialise again. They're not even mentioned once. Think how many paradoxes have occurred in Doctor Who that the Reapers should have intercepted. During the Moffat era, they should've popped up pretty much every episode!
What could (and should) have been a recurring enemy that demonstrated the dangers of time travel were instead relegated to a simple monster-of-the-week, forgotten by writers and fans alike. As the show began to focus on time travel more and more as a storytelling device, the Reapers were silently retconned out of existence, making one of the Ninth Doctor's best stories feel somewhat out-of-sync with the rest of the show.