> 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.
I find it too verbose. How about this alternative: "Every undocumented behavior of a system eventually becomes a contract."
Someone depending on it does not mean it becomes the contract.
Someone depending on implicit behavior means https://xkcd.com/1172/ and you shrug and break them anyway because the contract never required you to uphold the property. It's called contract after all, there's a mutual relationship, not a one-sided one.
Otherwise specifications would be meaningless and we would just tell people to read the source.
I find it too verbose. How about this alternative: "Every undocumented behavior of a system eventually becomes a contract."