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

Question

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...-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

metaloha · Accepted Answer

PHP for sure - used it for years, wrote multiple frameworks for various things, then migrated to JavaScript and the front-end and now I can barely remember how to write PHP :(

gladiatr72 · Answer

M$/PC/IBM DOS memory management driver configuration

superchroma · Answer

Probably more esoteric WPF stuff like bundling image resources in the DLL and defining control templates. I mainly remember how fiddly it all was.

tempxyz · Answer

Music I suppose. High school phase.