In Avengers: Endgame, the following is part of the dialogue between Bruce Banner (who traveled back in time) in astral form and the Ancient One:
ANCIENT ONE: I can't risk this reality on a promise. It's the duty of the Sorcerer Supreme to protect the time stone.
BANNER: Then why the hell did Strange give it away?
ANCIENT ONE: What did you say?
BANNER: Strange. He gave it away. He gave it to Thanos.
ANCIENT ONE: Willingly?
ANCIENT ONE: [long pause] Why?
BANNER: I have no idea. Maybe he made a mistake.
ANCIENT ONE: [long pause] Or I did.
I'm unclear on what mistake she is referring to. I suppose her mistake might simply be not immediately giving Bruce the time stone, but that doesn't seem profound enough for this dialogue.
Also it's odd that she knows Dr. Strange's future, but doesn't know she gives Bruce Banner the Time Stone. Maybe incorrectly perceiving the future is relevant to her mistake.
Question: What is the Ancient One's mistake?