In the mid 1970's, my company used mini computers made by a company called Modcomp. This was no ordinary company. For one thing, I remember hearing that one or two of their executives was wanted for accounting fraud. For another, the names for the Modcomp computer instructions were designed by an alien from outer space, and were generally less mnemonic than for other computers. Typical instruction names in those days were: JUMPL (jump to another address if result is less than zero); LB (load byte); ADD (add). In Modcompese, I'll bet you wouldn't guess what the SUM instruction did; that's right, it SU-btracted, storing the result in M-emory.
But that's not important now. Modcomp, unlike all of its competitors, built their minicomputers with chips that could easily be removed and replaced. (Take just a moment and savor the fact that in this, Modcomp differed from ALL of its competitors. Perhaps they knew something that Modcomp didn't know.) Chip replacement made it easy for Modcomp to upgrade machines and fix computer bugs, and they issued correction sheets all the time. Typically a bug was fixed by replacing a few chips, removing and rewiring a few wires. (There were printed circuit boards in the 70s, but typically every mini had lots and of real wires as well.)
At that time my company was developing software for a large oil company to control waste in a refinery plant. We bought the Modcomp mini-computers, sold them to our customer, and held on to them for many months while we developed the software for them. One day our customer mentioned a contract clause we had totally ignored: "You're keeping the computers up to date, right? All the upgrades and bug fixes?"
There was an awesome mound of fix-sheets in our director's desk drawer that we had never applied. He sorted through these, suspecting that some of them must be obsolete already. Eventually we forced a meeting with our customer and the director of the Modcomp fixups division, to decide what fixes must be applied, both now and in the future.
The Modcomp guy was proud of the intense rate at which his company issued fixes. He saw this process as insurance that the computers were as good as possible, although I suspect you will come to a different conclusion. At the meeting, discussion focused on the amount of time needed to take a computer out of service to apply each weekly batch of fixes. Finally our director confronted the Modcomp guy: "You're telling me that we'll have to take our computers out of service more than forty hours a week to keep them current?"
After some hemming and hawing, the Modcomp guy agreed. It was an astounding moment. Our customer agreed that we could apply only the most important fixes, to be determined by Modcomp, so that we could finish writing our programs for them.
I wonder how many fixes were added after the computer went into production at the customer plant. They wanted to run 24/7.