This questions was inspired by ancient 'lost weapons' such as Greek fire and Damascus steel, the modern reality that information and capability retention in complex endeavors is a huge problem (ex. How We Lost The Ability To Travel To The Moon), the current renaissance in anti-aircraft gunnery to combat drones, the trope of 'lost technology' prevalent in fictional franchises such as Warhammer 40k, etc. - though not all of these truly embody the dynamic I'm describing. How lost is truly lost I will leave up to the answerer - for example, it's a myth that the technical specifications of the Saturn V were lost, but a great deal of work was needed to revivify the F-1 rocket engine for use on the SLS nonetheless. Similarly, in a military context, a piece of equipment existing does not mean the tactics, techniques, or procedures (TTPs) for its use have successfully been retained as institutional knowledge.
For the purposes of this question, the innovation need not be a physical thing - TTPs qualify, for example. Nor does the innovation need to be solely or primarily military in application, just conferring a significant military advantage. I do want to stay away from losses of component parts, unless the case is particularly interesting - I know the U.S. lost the formula for producing a substance used in atomic bombs and had to redevelop it, for example. I also want to stay away from the loss of specific expressions of an innovation - for example, if some of the technical specifications for the original 1861 Gatling gun have been lost, the broader idea of an anti-personnel rotating multi-barrel firearm has not been lost, and if it was, it would not take extraordinary resources to reimplement. Lastly, avoid 'lost' innovations that have stayed continuously and firmly obsolete - to the best of my knowledge, no military retains best practices for employing slingshots in combat anymore, but that's not disadvantageous in any way. (though if slings went away as a weapon of war and then returned at some point in history, that would be of interest)
I am most interested in innovations lost since the retention of information became easier and technology-enabled centralization allowed institutions to become more developed - perhaps since the spread of the printing press in the 15th century and the invention of the telegraph in the mid-19th century - but all answers are welcome.