Hyrum Wright at Google observed that no matter what your API contract promises, given enough consumers, someone will depend on unspecified behavior.
Implications
- Internal implementation details eventually become the public API
- Changing anything—even bug fixes—can break someone
- This is why semantic versioning and deprecation policies matter
// You think this is internal?
function _privateHelper() { ... }
// With 10,000 users, someone is calling it directly.