Non-documented procedure
How many here do things at work that aren't documented? Did you learn it through co-workers? "When such and such happens, kick the rack on this corner....We've never been able to find that F-ing bad connection."
Stuff that didn't work rarely gets documented and when it is, it goes in the back of the cabinet until it's thrown out years later. When companies start axing older staff, they lose the search engine that knows about all those failed attempts. They not only lose some procedures, they lose the reason behind those fixes too.
"SCE to Aux". When Apollo 12 was struck by lightening as it lifted off of the pad its brains got scrambled. One engineer, John Aaron, recognized what was going on quickly since he'd seen it before on another rocket launch and knew how to get the systems back online. He was able to get the astronauts to flip an obscure switch and get back up and in control. A fresh-out engineer, no matter how talented, would not have been able to diagnose the problem and know the solution fast enough. Solving the problem fast had to be worth a couple of billion dollars or more than enough money to pay a whole bunch of salaries for years.