The 2008 episode "The Eleventh Hour"—the first episode of Series 5—ends with a dramatic confrontation between the Doctor and the leader of the Atraxi fleet on the roof of the hospital where Rory works. During the confrontation, the Doctor gets the Atraxi leader to scan his mind, then asks him three questions to convince not to destroy Earth. One of the questions, of course, is "Is this world protected?", followed by a montage from the Doctor's memories of various alien races from previous episodes that have menaced humanity.

During the montage, the Doctor tells the Atraxi leader, "You're not the first to have come here. Oh, there have been so many," while gesturing to the footage of the various aliens. If you look carefully, though, you'll notice that one of the species depicted in the clip is the Hath, the alien race from "The Doctor's Daughter". Though humanity did fight a war with the Hath, the Hath never set foot on Earth, and they never invaded a human world. The war started because humanity and the Hath both wanted to colonize the planet Messaline, and ended up fighting a battle over it.