Another fun play on this is the periphery in the Foundation trilogy. After the empire began collapsing and the central education systems started to fail and the imperial puppet governments dissolved into feudal realms, the engineers who maintained the reactors slowly became a priesthood.
After a few generations, the hereditary priesthood started to lose actual working knowledge of the old reactors, but they were such large and well built machines that they could operate for a thousand years without human intervention, so no one noticed. They stopped fixing things and began to slip into ritualistic practice to appease the machine and had generations of people believing that they alone were the ones keeping the reactors online when in reality they continued to decay and no one even knew.
