Hi!
Here's a little test case, first of all.

public unowned Date first () {
	/* In the real code, this value is returned by an external function
	 * so is actually unowned in this scope
	 */
	Date d = GLib.Date();
	d.set_dmy (5, DateMonth.APRIL, 2011);
	return d;
}

public unowned Date? second () {
	/* Date is stored on the stack and then a pointer to
	 * the stack returned :(
	 */
	return first();
}

int main(string args[]) {
	Date d = second();
	assert (d.valid());

	return 0;
}

Here's an extract from the C code:

GDate* test_second (void) {
	GDate* result = NULL;
	GDate _tmp0_ = {0};
	GDate _tmp1_ = {0};
	test_first (&_tmp0_);
	_tmp1_ = _tmp0_;
	result = &_tmp1_;
	return result;
}

.. so main() is returned a pointer to a now invalid stack object :(