Understanding the impact of NSubstitute's .Returns() static stack implementation

gistfile1.cs

using System;
using NSubstitute;
using NSubstitute.Exceptions;
using NUnit.Framework;

namespace Tests
{
    public class Class1
    {
        public interface Interface
        {
            int Method();
        }

        [Test]
        [ExpectedException(typeof(CouldNotSetReturnException))]
        public void Test1()
        {
            var x = Substitute.For<Interface>();
            0.Returns(3);
            // No calls on the stack so we can't set up a return value
        }

        [Test]
        public void Test2()
        {
            var x = Substitute.For<Interface>();
            var y = Substitute.For<Interface>();
            y.Method().Returns(5);
            // Pushes y.Method() call to the stack
            // .Returns pops the y.Method() call off the stack

            x.Method().Returns(y.Method() + 1);
            // Pushes x.Method() call to the stack
            // Pushes y.Method() call to the stack
            // .Returns pops the last item off the stack (the y.Method() call)

            Assert.That(x.Method(), Is.EqualTo(0));
            Assert.That(y.Method(), Is.EqualTo(6));
        }

        [Test]
        public void Test3()
        {
            var x = Substitute.For<Interface>();
            x.Method();
            // Push onto stack

            1.Returns(3);
            // Pop off stack

            Assert.That(x.Method(), Is.EqualTo(3));
        }

        [Test]
        public void Test4()
        {
            var x = Substitute.For<Interface>();
            x.Method();
            // Push onto stack

            "".Returns("");
            // Pop off stack - it doesn't inspect the type; it's just a pure stack

            Assert.Throws<InvalidCastException>(() => x.Method());
        }
    }
}