That has happened quite often in the past [1].
That has happened quite often in the past [1].
Petrification: no real loss to society but I will concede that this appears to be an example of actual loss of "progress". Very few people are saying "oh diety, if only I could preserve this animal with perfect color and texture (but discarding all other characteristics).
Greek fire: the history of when this knowledge was lost is unclear. Also it was intentionally kept secret. Certainly we have functionally similar tech now. I'll give 50% credit.
Panjagan: We don't even know if this was a weapon or a technique? Everything about this is incredibly vague.
In summary, this has happened approximately 1.5 times in the past. Not "quite often".
[1] https://news.mit.edu/2023/roman-concrete-durability-lime-cas...
Similarly expertise in plutonium manufacturing has been lost, and now that countries have run out they are re-learning how they did it 40 years ago.
https://en.wikipedia.org/wiki/Fogbank
Edit: I see someone beat me to the punch with Fogbank. For another example, look to the F1 rocket engine that took us to the moon. Despite having the actual engineering documents, we just don't have the manufacturing skill to rebuild one: https://apollo11space.com/why-cant-we-remake-the-rocketdyne-...
The F1 is particularly interesting, because if we'd undergone a program in 1985, we probably could have restarted production, maybe even 1995 - but every decade another n% of the engineering knowhow sluffed off into the permanent dirt nap until there wasnt enough left.
Another recent example was when Jay Leno had a new heat exchanger made for his Chrysler Turbine car, and they were able to call some retirees in to help make a new one.
Us Europeans learned that lesson the hard way three years ago and we haven't made much progress ever since the first Russian boot set foot on Ukrainian soil. 100k artillery shells don't sound like much of a thing... but apparently it is.
Really fascinating bit of trivia though!
[0] https://en.wikipedia.org/wiki/Sloot_Digital_Coding_System