The update that broke the printer

A school's printers stopped working after an automatic security update. The update was correct, necessary and installed on time. It also removed support for an old network protocol that the printers were the last devices to use.

This kind of failure is structural rather than careless. A system has dependencies that nobody has written down, and the ones that break are reliably the ones nobody knew about. The printer's protocol was not a secret; it was simply boring, undocumented and working, which is the exact profile of a dependency that will surprise you. Technicians use the phrase 'inventory before upgrade', and in practice it means something modest: a list of every device that talks to the network, what it talks to, and who will complain when it stops. The list is tedious to build and goes out of date immediately, which is why the useful version is not a document but a habit — a question asked in the week before any upgrade, to the people who use the equipment rather than the people who manage it.

The school's eventual fix combined three things. The printers were replaced over two years rather than one week, with the oldest first. A test machine received every update one week ahead of the rest, which is cheap and catches most problems. And a line was added to the maintenance log naming the person who had asked 'what else uses this' and what they were told.

That last item sounds bureaucratic. It is the one with the longest life, because in three years nobody will remember the printers, and the next person will inherit the question rather than the answer.