The Doctor and Clara in “Before the Flood”

This isn't a potential future. This is the future now. It's already happened. The proof is right there in front of you. I have to die.

No. You can change things.

I can't. Even the tiniest change, the ramifications could be catastrophic. It could spread carnage and chaos across the universe like ripples on a pond. Oh, well, I've had a good innings. This regeneration, it's a bit of a clerical error anyway. I've got to go sometime.

Not with me! Die with whoever comes after me. You do not leave me.