Take some code like

```
if (person.IsMale()) {
    doGuyStuff();
} else {
    doGirlStuff();
}

```

Should this be written to explicitly check if `person.isFemale()`, and then add a new else that throws an exception? Maybe you're checking values in an enum, or something like that. You think that no one will add new elements to the enum, but who knows? "Can never happen" sounds like famous last words.