it's a machine that was built many years ago. they realized they had all but one of the parts, a pretty important one, but they found a way to rig it to temporarily work in its absence.
except years go by. and the machine is still doing its job, sure, but no one ever thought to do maintenance or locate that integral part, or even to find a decent replacement. it's been too long, anyway, there's no use in trying to fix a machine that's already been running.
all the current parts look in good enough shape. or at least they seem normal from a distance, if you squint. and it still works.
the way the machine works is causing damage to the other parts, though it is very slow and near imperceptible. no one bothers checking for damage. there's no symptom of any problem.
the machine will eventually fall apart. it's learned to live without that single key piece, and it will die without it, regardless of if it's found at all.