Speaking during last night’s Facebook Live Q&A, he revealed that the final episode, which is due to air next Saturday, is not yet finished.
After being prompted by an audience member to give the start time for next week’s instalment, the 55-year-old made a joke about his slip-up.
Moffat revealed: “We haven’t actually finished episode 12 yet. We haven’t seen the finished version so I have got things to worry about before we worry about when we put it out.
“It could be Sunday, it could be the following Thursday,” he teased.
Last night’s tense episode saw the return of John Simm’s version of The Master, leaving fans shocked.
The popular incarnation of the Doctor’s (played by Peter Capaldi) arrived at a crucial time having just sent companion Bill Potts (Pearl Mackie) to her death after being turned into a Mondasian Cyberman.
Moffat appeared to confirm that Whovians have seen the last of Bill, joking: “She’s a Cyberman from now on. She’s one of those ones in the trailer actually.
“She was just killing people. That’s the way it is sometimes,” he teased.
As Mackie discussed the “emotional” finale, fans thought that was her confirming she was leaving the series for good, but a production source quashed rumours, telling Radio Times: “Pearl was answering about the scene where Bill appeared to die before being taken away for ‘treatment’ at the hospital, where viewers later discovered that she was eventually converted into a Mondasian Cyberman.
“This is not a confirmation either way,” they tantalisingly added.
Doctor Who concludes next Saturday on BBC One at 6.45pm.