Take the follwing example:

class Foo
{
}

class Bar : Foo
{
	void Test()
	{
		print("Test was called");
	}
}

void BugTest()
{
	ref@ x = Foo();
	Bar@ y = cast<Bar>(x);
	if (y is null)
		print("y is null, as expected");
	else
	{
		print("y is not null, calling Test() now:");
		y.Test();
	}
}

Here, the explicit cast to Bar should return null as expected, but it doesn't, because it only checks whether the types are derived from each other. Calling y.Test() can even cause a crash here.

If you explicitly cast the ref to Foo first before casting that to Bar, it will return null properly.