In the scenario above FlatBoat has 2 natural ids. One which is immutable and the other is mutable. In the unit test I update the immutable natural id and it doesn't give an error.
We are currently upgrading from hibernate 3.3.2.GA. On 3.3.2.GA the commit fails because the natural id is immutable and the test passes. On 4.3.11 the test fails.