In Avengers (2012), there is the following dialogue:
NATASHA: Clint, you're gonna be alright.
CLINT BARTON: You know that? Is that what you know? I got...I gotta go in though. I gotta flush him out.
NATASHA: We don't have that long, it's gonna take time.
CLINT BARTON: I don't understand. Have you ever had someone take your brain and play? Pull you out and send something else in? Do you know what it's like to be unmade?
NATASHA: You know that I do.
Why does she say this? When and by whom was she "unmade"?