Fourth Wall

When the Doctor wished "a Happy Christmas to all of you at home", (TV: "The Feast of Steven") had just awoken from an alcohol-induced sleep, and she staggered through the now open Fourth Wall to an snowy area where her beautiful bus stood waiting. (PROSE: From Wildthyme with Love)

Behind the scenes
The fourth wall, initially a piece of theatre terminology, has been "broken" numerous times in the DWU; however, From Wildthyme with Love marks the first time the fourth wall itself has been mentioned in-universe.

However, despite the fourth wall being acknowledged (as well as the exact circumstances around it from The Daleks' Master Plan), Iris Wildthyme escapes through it to another in-universe location. This is yet another story which implies the existence of an in-universe version of the Doctor Who TV series.