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.
I had to look that up, just to make sure I understood it correctly, since it's one of those terms we so often take for granted, like a dusty old heirloom that's been passed from generation to generation. And though you get the basic gist of it, the phrase only comes to mind because you've heard it somewhere before. You know how it is. You make a mistake. You realize you made the mistake. And when the sting and the embarrassment of it finally subsides, you chuckle quietly to yourself and say, "Well, hindsight is 20/20, after all," as if that's the perfect capper to whatever learning experience you're facing at the moment. Or maybe I'm alone in thinking this way, I don't know. You're squinting your eyes at me now, confused. I can see it.