Actually, I had one just last night. I was starting an anthology on Buddhist philosophy (Empty Words) which I had downloaded from library.nu, and the very first essay was arguing that Nagarjuna's Verses on the Heart of the Middle Way and Sextus Empiricus's Against books espoused a similar skeptical approach to metaphysical commitments (drawing on Kripke & Wittgenstein) - the exact approach I had mused upon at length before, with some of the same passages I would have chosen.

'Huh', I thought, 'I really should have expected someone to have thought that before, but I guess I didn't because I was surprised and angered (since "my" idea had been stolen) to realize where this essay was going.'

Fortunately, I hadn't gotten around to carefully studying and making notes towards an essay, so I didn't lose too much time to my failure to check whether the idea had already been done. (Unlike the time I re-invented longevity insurance, eg.)

The illustrations I find most vivid are from the past. Pick some particular technical idea that seems like a "big deal" to you, and then look at its penetration into academic and popular consciousness over time. Diagonalization arguments are a reasonable example. Cantor published over a century ago. The argument is simple enough to explain to an interested 11 year old in less than two hours and repairs basic confusions that many reasonably smart people have about "infinity"... and it remains nearly unknown among non-mathematicians to the present day.

Now imagine the tiny fraction of the population in 1910, 1930, and 1950 who knew about it, and the tricks they could do that "mere mortals" could not. That kind of stuff propagates slowly. Maybe faster now, what with the internet and paywalls slowly coming down and Wikipedia and so on? But still pretty slowly. Kolmogorov complexity has been an available thought for half a century already!

It would not surprise me if every insight needed for AGI had already been published somewhere already, but the separate ideas have just not yet been noticed, collated, synthesized, and reduced to practice. I find this thought sobering.

One of the real tricks is to know what the keywords are, and the best way I know to do that is to participate in academic specialties enough to pick up the "chalk board culture" of a new research group. Grad school (in addition to all the other stuff) is, in some sense, joining a research group and hanging out with them enough to pick up their chalk board culture. Not having access to this is one of the problems that an autodidact trying to make it with book learning and nothing but book learning can run into. Access to the chalk board cultures is partly valuable for helping you see what other smart people think are important things to have read. There's other value as well (like the way a chalk board culture is sometimes a locus of knowledge production), but the advice on keywords and authors is a good chunk of the value.

Try a bunch of queries; read good-looking papers, not for their results, but their discussion of background information. There's no real good solution to the problem of known unknowns: "I'm sure this has been researched, but what's the exact phrase or name it's been given in academia?" I suspect this is one of the benefits of being in the community - when you discuss something or mention an idea, they can say 'ah yes, X' and now you have the much much easier problem of a unknown known which you can just punch into Google Scholar and get to work.