Personal project a while ago. Had an idea in my head that I needed to rewrite using a particular language feature but had been putting it off because I couldn't quite wrap my head around the implementation details.
Eventually decide to sit down and plug away at it. Find the code already uses the language feature.
I had either written it with that language feature in the first place or had been back at some time I don't remember and done the work I didn't think I was capable of doing.
I could still be convinced that it was done by pixies or the whatever might be the programming equivalent of shoemaker elves.