This reminds me of how the British Navy forgot the cure to scurvy after 150 years. http://idlewords.com/2010/03/scott_and_scurvy.htm

Like when Romans invented the scalpel forgotten only to be reinvented in the 19th century (stretching a bit since IIRC the Arab kept using it) ?

That a subject that never cease to amaze me. Civilizations rises and fall and with them knowledge. To me, Dan Simmons kind of use that in Ilium and Olympos as background. What will happen when our digital civilisation falls ?

‘Documentation’ was not always as cheap as it is today. If it takes hours to engrave a few words in a stone board or if a single page of paper is worth more than a simple meal, you think twice about what you ‘document’. Chances are that such common practices were considered that obvious that they didn’t need documentation on their own, similarly how you would likely not include ‘plug the computer into a power supply’ when asked to tell a relative how to set up theirs.

Why document something that everyone does? History is captured in sources like published news, government statistics, personal letters, etc. A personal behavior that is widely understood would not necessarily be newsworthy enough to capture in any of those places.

This is a real problem for historians: unusual occurrences are usually well-documented in permanent media; everyday life usually is not.