twenty-twenty hindsight

from The Collaborative International Dictionary of English v.0.48

hindsight \hindsight\ n.
   understanding the nature of an event after it has happened;
   as, hindsight is always clearer than foresight.
   [WordNet 1.5]

   {20-20 hindsight}, {twenty-twenty hindsight} Perfect
      understanding of an event after it has happened; -- a term
      usually used with sarcasm in response to criticism of
      one's decision, implying that the critic is unfairly
      judging the wisdom of the decision in light of information
      that was not available when the decision was made.
      [PJC]