I didn’t know this was out there, a funny-yet-astute-and-accurate observation from a software engineer at Google:

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.

Even the tiniest, trivial behavior of your API is not only desirable by some user out there, it’s critical. We have a joke internally that goes something like:

“Don’t change that {API response | setting behavior | data storage format}, my entire business model depends on it!

The only edit I would make to the Law is that the quantity of users required to start uncovering emergent, unplanned, and assumed behaviors is much smaller than you would think.

We don’t have near the scale of API or library usage as Google, and we encounter this regularly with our APIs.