Poka-Yoke: Make the Mistake Unsayable

People will always make mistakes. That is not the problem worth solving. The problem is letting a mistake become a defect.

Shigeo Shingo, a Japanese industrial engineer, worked this out on a switch assembly line in 1961. Workers kept forgetting a spring. The fix was not a reminder: the job was split so the worker first laid both springs in a dish, then fitted them from the dish. A spring left over was the error announcing itself, before the unit could move on.

The dish is a device. "Please remember the spring" is not.

The line that does most of the work

A comment, a docstring, a wiki page, a review checklist, or a line in an instructions file saying "don't do X" is not a poka-yoke. It is training, and training degrades. A device does not. If your fix relies on someone remembering something, keep going.