Hyrum's Law

Spolsky's "Law of Leaky Abstractions" embodies consumers' reliance upon internal implementation details.

"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.

In other words, an interface should theoretically provide a clear separation between consumers of a system and its implementers. In practice, this theory breaks down as the use of a system grows and its users start to rely upon implementation details intentionally exposed through the interface, or which they divine through regular use.