The problem is that people commonly don't even realize they're depending on implementation quirks. For example, they write code that unintentionally depends on

The problem is that people commonly don't even realize they're depending on implementation quirks. For example, they write code that unintentionally depends on some distantly-invoked async tasks resolving in a certain order, and then the library implementation changes performance characteristics and the other order happens instead, and it creates a new bug in the application.