20/20 Hindsight

20/20 是正常視力的意思。 If yoiur eyesight is 20/20, that means your eyesight is perfect.
Hindsight 20/20 或 20/20 hindsight 解百分百事後聰明/事後孔明，是自嘲或嘲笑別人時用。
Hindsight, looking back on what has already happened, allows us to see clearly the course of action we should have taken,. So if you nag me with what I should have done yesterday, I will remind you that your hindsight is 20/20.

please refer to the below yahoo answers

http://au.answers.yahoo.com/question/index?qid=20070108211210AAR6o6v