1851 incident
From Tardis Wiki, the free Doctor Who reference
The 1851 incident, was the name given by The Secret Lives of Monsters to the event in which the Cybermen attacked London in 1851. (TV: The Next Doctor, PROSE: The Secret Lives of Monsters)
The Eleventh Doctor theorised this event was later erased by a crack in time created by total event collapse, due to the apparent lack of memory of it. (TV: Flesh and Stone)
