Which is correct - "Whose mistake was it?" or "Who's mistake was it?"?

Whose is the correct answer.

Whose is a possessive adjective describing the noun 'mistake'.

Whose mistake was it?

Whose book is this?

I want to know whose book is on the table.

Do you know whose jacket this is?

Who's= contraction.... who is

Who's going to the game this Saturday?

Who's working tonight?

Whose

Who's is the Combination of "Who" and "Is". To say "Who is mistake is it anyways" does not make sense at all.

Whose.

Who is mistake was it doesn't make any sense.

"Whose" is correct.

"Who's" is a contraction of "who is" and "who is mistake was it?" does not make any sense.

"whose" is correct here. "who's" is short for who is.

Who's I think xD