In the old way of doing things, we treat our servers like pets, for example Bob the mail server. If Bob goes down, it’s all hands on deck. The CEO can’t get his email and it’s the end of the world. In the new way, servers are numbered, like cattle in a herd. For example, www001 to www100. When one server goes down, it’s taken out back, shot, and replaced on the line.
With a sufficient number of users of an API, it does not matter what you promise in the contract: all observable behaviors of your system will be depended on by somebody.
Taken to its logical extreme, this leads to the following observation, colloquially referred to as “The Law of Implicit Interfaces”: Given enough use, there is no such thing as a private implementation. That is, if an interface has enough consumers, they will collectively depend on every aspect of the implementation, intentionally or not. This effect serves to constrain changes to the implementation, which must now conform to both the explicitly documented interface, as well as the implicit interface captured by usage. We often refer to this phenomenon as "bug-for-bug compatibility."
At this point, the interface has evaporated: the implementation has become the interface, and any changes to it will violate consumer expectations. With a bit of luck, widespread, comprehensive, and automated testing can detect these new expectations but not ameliorate them.