Apparently we not only lose technology we also lose history. So things that were lost and reinvented usually in a different way: The wheel, Dome building (got reinvented twice 1k years in between), printed letters (3k years), self healing concrete (not sure if has been reinvented since the romans), astronomy calculators (1.5k years? not sure is there was an antikythera mechanism in the rennaisance), steam operated opening doors (2k years, we went directly to electric).

These are of the top of my head mostly covered by HN posts. Especially the antikuthera mechanism the loss was that entire that even the memory of this mechanical calculator or even the existence of other such calculator was lost. Also lost apparently was the process of creating an H bomb that"s for good but probably it has been secretly reinvented by now.

This is a clear case of a discussion appearing as it has a tech content and in fact its historical. Statistically and given tge infinite creativity of people infinite things will have disappeared, through a process of randomness, usefulness and politics a few prevail. The wheel was forgotten because it was useless. It required working economies, trade and usable roads, even dirt roads. As for printing letters if that prevailed when it was invented 2-3k years back we could have reached the stars by now.

In the times its even easier to lose technologies, they are not intuitive anymore, they are stored in miniture formats easily forgotten and stored easily corrupted and fail media, guarded behind laws, encryptions and interests. Medical technology is our lifetime's major culprit.