"Planet of the Hats" was a hypothetical planet that Donna Noble said she'd be ready for given the hat box she brought with her in the TARDIS. (TV: Partners in Crime)
The Doctor, in his twelfth incarnation, claimed that such a planet did not exist, but Bernice Summerfield and Keri asserted that not only did it exist, but that they had visited it. (PROSE: Big Bang Generation)