My favorite thing is stumbling upon a piece of project documentation, exclaiming "who the hell wrote this crap?", then discovering that it was me.
Only once have I looked at something and thought, "cool solution, I would never have thought of that", but apparently I did.
That is especially frustrating.
“hhh: There's probably a better way to do this, but that is a problem for you, dear reader, who is likely future me.”