Ask HN: What were you once expert, but now you can hardly recall what you knew?

30 years in - there are various areas where I was the SME/expert on certain things, but if I were to be interviewed on them, I would sound completely like I never knew anything about it...

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What are some areas where you had intense focus, drive and knowledge, but years later - you can now barely speak as an authority on the subject?

"I've forgotten more about [subject] than you would ever know] trope