How is “in mistake for” used?

In What If (2013), one line is: “That was a mistake.”

Which movie is “in mistake for” from?

The clip at the top of this page is from What If (2013).

Idiom

thinking that sth is sth else

Heard in 524 movies

in mistake for sth
thinking that sth is sth else
Children may eat pills in mistake for sweets.

Definitions from the Oxford Advanced Learner's Dictionary, 9th edition

That was a mistake.

Said in 8 films · #1