For example, the other day someone asked on stackoverflow why a particular Java class to calculate a difference between two dates was returning a weird result when they asked for the difference in days. Turns out, the class would calculate the difference in years AND months AND days, so if you asked for the difference between June 23 2018 and June 23 2019, the getDays method would return 0.

While this is something you would find in a tutorial on how to calculate a difference between dates in Java, it's much more expedited to just go and read the reference. Works wonders when you get inconsistent results in C for example, since you're probably invoking undefined behavior which is documented in the standard (i.e., the reference).

As a sort of counterpoint for this particular case, the documentation for that class didn't make how the class was meant to be used clear at all. But in the general case I think it applies.