Deck 7

From Tardis Wiki, the free Doctor Who reference

Deck 7 was the seventh deck on the Doctor's TARDIS. It had a waste tank that could be evacuated by pressing a button on the TARDIS console.

Unaware of what some of the TARDIS controls did, the Doctor had a habit of pressing the button while piloting the TARDIS, inadvertently emptying the tanks without his knowledge. Upon being notified of the button's function by River Song, the Twelfth Doctor stated that he "better avoid Deck 7". (TV: The Husbands of River Song)