Is it a bad practice to write (hopefully) unreachable code?

Let's say you want to union two intervals. The code could look like this:

```

def union(self, other):

    # Capture special cases
    if self.is_empty():
        return other
    elif other.is_empty():
        return self

    # Standardize - after this step, the other.left is left of self.left
    if other.left > self.left:
        other, self = self, other

    # Go through all cases
    if other.right  self.right:
        # other is a superset of self
        return other
    else:  # noqa
        # This should never happen
        raise NotImplementedError("Can't merge {} and {}".format(self, other))

```

Note that at the very bottom, there is unreachable code. At least I hope it is. If it isn't, then there is a bug. Making an error throw early is good practice. Unreachable code is bad. So ... is this good or bad?