Monday morning quarterback.
This idiom or expression means:
A person who finds faults, and blame others, AFTER everything has been done.
A. I would have done the whole thing differently and avoided the present mess.
B. Yeah, it’s easy to be a Monday morning quarterback!!
History:
Most (American) football games are held on Sundays. This leads to lots of Monday morning conversations among football enthusiasts at their work place, during which everybody comments on how the games should have been played. Hence the term “Monday morning quarterback.” Keep in mind that, in American football, the captain of the team is the quarterback.