I do this with documents and help articles I've published. "Here, let me show you how to do this." 6 mos later: "How the heck did I do that?... oh yeah. That makes sense, neat."
I had a similar thing happen after watching a YouTube video a couple weeks ago. I was going to leave a comment and looked through the comments first to see if anyone felt the same as I did. I read a comment that was nearly word for word what I was thinking only to realize it was mine.
I have got the "you can't upvote your own answer" warning several times over the past decade. It's a wierd mix of delight and embarrassment.
What's worse is when you find your own answer and don't even remember writing it!
Worse still is finding entire old projects with your name on them and not remembering them at all.
Happens to me with my own code. "What the hell was I thinking?" Isn't uncommon when I find I got a bit too clever on something. Over the years I've learned to be less clever and more literate in my coding style.
Why did you spend hours digging into something you already knew the answer to?
They didn't know it any longer. They are using StackOverflow as a second brain, offloading knowledge to it.
Pensieve.
Because "knew" is past tense. There are a lot of things that I once "knew" but no longer "know", and it's nice when that past knowledge is documented somewhere.
Solving a problem once doesn't guarantee it's added to your knowledge. Learning needs repetition, if you solve that problem once a year, you'll learn it very slowly.
Not OP, but... more often than I care to admit, past me is perfectly aware of the answer to this problem, but present me has long since forgotten.
Knowing something at some point in the past does not mean you currently remember it!