With sufficient users, every observable behavior of your system will be depended upon by somebody.
Hyrum Wright's law from Google. If a function returns results in a particular order by accident, someone depends on that order, and changing it breaks them regardless of what the documentation says.
In practice
- An internal API that returns results alphabetically by accident has four teams depending on the ordering by the time anyone documents it.
- A form that tolerates a trailing space in a reference number has customers who now always type one.