“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.”

“Roughly 80% of consequences (output) come from 20% of causes (input).” In computer Science, this principle can be applied to a variety of scenarios. For example, “Nearly 80% of the bugs are found in 20% of the code”, “Nearly 80% of the traffic is generated by 20% of the clients” etc.