- Scientists still debate exactly how the pyramids were built. They have a pretty good idea, but don't know exactly.
- More recently, we're not sure we could even recreate the engines that powered Apollo and took us to the moon. It appears that some of the critical knowledge was institutional, passed from one to the next, and all the engineers are gone.
And look at any tech company now. Sometimes when one person leaves, stuff has to be rewritten because no one knows how it works. My favorite example of this, which I experienced personally, was that no one knew how the "send money" code at PayPal worked for the longest time. It was written in a language that the lead engineer made up (and then he subsequently left). Everyone knew it worked, but no one knew how. They had to eventually rewrite the whole thing from scratch, but not after creating a fleet of emulators to keep it running.