What I usually say after I mess up big and realize my mistake(s): "Lesson learned." Of course, I actually have to learn the lesson, or else it was all a waste of time.

Wiki is like one big -- or thousands of little -- lessons learned.
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Why do we use this term?  It seems to me that the more we shove our experiences into boxes, the less we can gain from them?

''You're right. The whole patterns thing is a waste of time too.''  Totally.
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See also LessonsLearned