This seems to happen a lot. One people seem to keep chasing is Damascus steel.

Even in more recent history, too. I like to research fine weapons. Apparently the process Colt used on their pistols to color and shine them is completely undocumented and now unknown. Similarly, it's said there are not enough people alive that know how to handfit a revolver. I'm not sure how true the latter is, but really makes you wish Wiki or the internet existed further back.