Either way, it's a mistake (4)
I believe the answer is:
boob
I believe this is a double definition.
'either way' is the first definition.
'a mistake' is the second definition.
(I've seen this before)
'it's' is the link.
(Other definitions for boob that I've seen before include "Embarrassing mistake" , "Blunder (colloq.)" , "Error" , "Slip" , "Mistake; breast (sl.)" .)