Flesh and Stone
The Doctor refers to the events of the previous Dalek invasion of Earth (in "The Stolen Earth" and "Journey's End") when talking to Amy about the Daleks, becoming concerned when Amy does not remember these events. At the end of the episode, a crack is shown in the wall behind where the TARDIS had been parked. The crack, having first appeared in Amy's bedroom when she was a little girl, has been a recurring theme; in "Flesh and Stone" the Doctor discovers it has the power to erase things from existence, the reason Amy cannot remember the Daleks.