Degraded mode

Any system with enough parts always has some of them broken, worked around or quietly out of spec, because fixing everything would mean stopping everything. The system works anyway, which is why nobody notices, and why the eventual failure is never caused by one new fault but by an old one meeting another.

A system small enough to hold in one head, a new system on its first day, or a checklist short enough that every item actually gets done; none of these stay that way for long.

— Richard Cook, 'How Complex Systems Fail', 1998