Back and there again is the seventeenth level of Chapter 4.
The TARDIS doors slammed shut. With what sounded like a large and hollow "pop" the Twelfth Doctor reappeared, shocked, in the middle of the console room.
Twelfth Doctor: Whoa Nellie!
He pats himself all over, checking he's really all there. He points at the older River Song.
Twelfth Doctor: You. Hair. You're taking some serious blame for this one.
He points at the younger River. Pushing a lever slowly down, the lights dim, the TARDIS engines go up an octave - subtle, but the roar is so all-encompassing in the crowded control room that everyone notices and looks around.
Twelfth Doctor: And you, younger one, your timeline is at fault too. Now, if I can just boost the power enough...
The Eleventh Doctor appears, horizontal, several feet off the ground, and falls unceremoniously. He wipes his jacket as he lifts himself off the ground.
Eleventh Doctor: I always meant to build a robot to keep this place clean. Keep boosting the power!
Professor River Song: You're using the TARDIS to stabilize your time line?
Eleventh Doctor: The web of time is collapsing, and all of me being here along with both of you, given how intertwined our threads, our timelines are, they just...
The Tenth Doctor walks into the control room through an adjacent door.
Tenth Doctor: The timelines just snapped, temporarily. But the TARDIS can hold everything together, at least we think she can.
One by one the incarnations of the Doctor reappeared, until...
Eleventh Doctor: There are places in space and time where the destinies of two people collide. Places that are so important that the stress they put on the web of time is incalculable. Thirteen of me is bad enough, but being at the library with two versions of River Song...Let's just say we need to be a lot more careful in the future.
The Eleventh Doctor walks up to Professor River Song.
Professor River Song: I looked after them, we went on a field trip to the library.
Eleventh Doctor: I never doubted for a second you would take care of them.
Professor River Song: I checked out a book!
Eleventh Doctor: Then it's time to devise a plan. Brigadier, follow me!