The problem in general is a language feature silently increasing the API surface. In this specific case the function was already callable with two parameters,

The problem in general is a language feature silently increasing the API surface. In this specific case the function was already callable with two parameters, without the library author's intent. Another example to the general rule is the fragile base problem: every method is a customization point by default. These "convenience" features can easily turn into headaches like this for library authors.