The routine that has outlived the reason it was built for

Most stale routines were once entirely sensible, which is precisely why they are hard to spot.

A structure gets built for a set of circumstances: a job with a certain start time, a child of a certain age, a house with a certain layout, a body with certain constraints. It works, it settles, and then it stops requiring attention, which is the whole point. Meanwhile the circumstances move. The job changes, the child grows, you move house. The structure does not move with them, because nothing tells it to. It keeps running competently against conditions that no longer exist, and it does so quietly enough that nobody notices for years.

These are difficult to detect because they are not failing. A failing routine complains: it is uncomfortable, it produces friction, you think about it. An obsolete routine produces no signal at all. It runs smoothly, delivers something you no longer need, and consumes time you would use elsewhere if you knew it was available. The very qualities that make a good routine, which is smoothness and invisibility, are the qualities that let an obsolete one survive undetected.

The only reliable remedy is deliberate inspection at intervals, since the thing will never raise its hand. Pick a repeated action and ask what problem it was originally built to solve, then ask whether that problem is still present in the same form. Sometimes the answer is yes and you carry on with more confidence than before. Often the answer is that the problem was solved elsewhere years ago, and the routine has been faithfully guarding an empty room.