1851 incident

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)

Background
When the TARDIS fell through the Void, the Tenth Doctor briefly visited a parallel universe home to a race of Cybermen who began as humans on Earth. (TV: Rise of the Cybermen/The Age of Steel) These Cybermen were eventually confined to their Cyber-factories by the Preachers but, by following the path of a Dalek Void Ship, were able to manifest themselves in the Earth of the Doctor's universe. Intending to seize the planet, the Cybermen came into conflict with the Daleks in the Battle of Canary Wharf which ended when the Doctor sent both armies into the Void. (TV: Army of Ghosts/Doomsday)

As the Doctor later became aware, (TV: The Stolen Earth/Journey's End) a greater battle weakened the walls between universes, destroying most of what then inhabited the Void, though a number of Cybermen survived by falling through the dimensions back through time to Earth. They did so using a Dimension Vault which they had stolen from the Daleks along with a database. (TV: The Next Doctor)

Aftermath
Though the Doctor believed these to be the "last of the Cybermen" from Pete's World, (TV: The Next Doctor) one source speculated that the Cybermen of the Cyber Legions which operated in his universe wre the result of cross-pollination between the parallel Earth Cybermen and those who were descended from the Cybermen of Mondas. (PROSE: The Whoniverse)

The Eleventh Doctor theorised that 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)