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Hindsight is perfect => 20-20 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.


http://www.thefreedictionary.com/20-20+hindsight

It mean that you can look in the past (hindsight) and see what you did wrong. Perfect vision is 20/20. So as you look back, you're able to see "perfectly" what you should have done. "Hindsight is 20/20" 

Read more: http://wiki.answers.com/Q/What_does_it_mean_to_have_20_20_hindsight#ixzz1HcT4HOvF