Architecture & practice·Risk, blast radius and change
you changed the order of a JSON response and a customer's integration broke, and nowhere did you promise an order.
Hyrum's law
Also calledlaw of implicit interfaces
With enough users, every observable behaviour of your system will be depended on by somebody, regardless of what you documented. Response ordering, timing, error message wording, the number of decimal places — all of it becomes contract by use. It is the reason internal-only rarely stays internal-only and the reason a deprecation plan is a negotiation about actual usage rather than about what the specification said.