I finished off an old C64 game called Shogun that, if you look it up, people will say it's impossible to finish. I don't think I saw another person reporting having finished it. And it didn't have any guide that I've ever seen. (It released with a manual that explained a lot, just going by faint memory, but I've never seen a scan of it, so I don't remember how useful it was. My memory from the '80s was that it wasn't very useful.)
But I just figured it out playing it again, knowing what I know now about game systems, then made a guide and a recording of finishing it that I think is still out there on some forum somewhere. I can't even remember where that is now. But that's an example of coming back to a game ~40 years later to finish off without even being sure it could be finished.