Test.cs

[Test]
public void SimpleTest()
{
	var calculator = Substitute.For<ICalculator>();

	int z;
	calculator.Add(1, 2, out z).Returns(x => { x[2] = 2; return 3; });
	calculator.Add(4, 5, out z).Returns(x => { x[2] = 20; return 9; });

	Expect(calculator.Add(1, 2, out z), Is.EqualTo(3));
	Expect(z, Is.EqualTo(2));

	Expect(calculator.Add(4, 5, out z), Is.EqualTo(9)); // Expected 9 but was 0
	Expect(z, Is.EqualTo(20));
}

This is a known issue. Problem is that Add(4,5,z) is configured when z=0 (its default value). Then during the test, Add(1,2,z) sets z=2, and the next call is Add(4,5,out 2), which is not configured. If you add z=0; to line 12 the test should pass.

Thanks!