The showrunner suggests he may written himself and the Doctor into a tight space ahead of the series finale.

The finale of Doctor Who series nine looks set to pose a serious challenge – not just for the Doctor but for writer Steven Moffat too.
Moffat suggests he may have written himself – and presumably the Time Lord – into a tight space in the previous episode, admitting he’s currently “in a state of panic” over it.
“I’ve got a script where I’ve set myself a ludicrous challenge,” Moffat told interviewer Jaci Stephens during a Bafta talk in New York earlier this month. “When episode 11 comes up in the next series of Doctor Who, you’ll see just how ludicrous the challenge I’ve set myself is. I’m in a state of panic about it.”
If episode 11 does indeed form part of a story with the series finale it would lend weight to the idea that the whole of series nine could consist of two-part adventures.
We already know that it will kick-off with The Magician’s Apprentice/The Witch’s Familiar, featuring a returning Michelle Gomez as Missy, which will be immediately followed be another as yet unnamed two-parter penned by Being Human’s Toby Whithouse.
Episodes five and six, The Girl Who Died and The Woman Who Lived, come next and star Game of Thrones’s Maisie Williams. And after that are two episodes both directed by Peter Harness and directed by Daniel Nettheim, suggesting they form a complete story too.
If parts 11 and 12 also come as a package, that would leave only nine and ten that we don’t yet know about.
Moffat said viewers had been getting “too acquainted with the rhythm” of single episodes and that penning series eight’s two-part finale Dark Water/Death in Heaven had convinced him it was time to mix things up a bit.
“Writing the first two-parter that I had done in years I just thought, ‘I’m liking this. This feels more unpredictable.’ Because you don’t know how far you’re going to get through the story…”
“The rule I’ve got is that you won’t be absolutely certain [from the story and tone] whether a show is going to be a two-parter or not,” he told Doctor Who magazine. “And with each of the two-parters we’re doing, there’s a substantial difference between the two halves.”
News Source: Radio Times.