Verify is not capturing a classes state at the time of invocation.

[djseng]

I expect this test to pass:

    [TestClass]
    public class IsThisABug
    {
        [TestMethod]
        public void SaverSavesTwice()
        {
            var mock = new Mock<ISaver>();
            var saver = mock.Object;
            var thing = new Thing { Value = "A" };

            saver.Save(thing);

            thing.Value = "B";

            saver.Save(thing);

            mock.Verify(s => s.Save(It.Is<Thing>(t => t.Value == "A")));
            mock.Verify(s => s.Save(It.Is<Thing>(t => t.Value == "B")));
        }

        public interface ISaver
        {
            long Save(Thing thing);
        }

        public class Thing
        {
            public string Value { get; set; }
        }
    }

The first Verify call fails, output says that Value == "B" was executed twice.

[stakx]

This is by design. Moq observes the usual .NET equality rules (object.Equals), so by default, reference types are compared by reference/identity. You can either turn your type into a struct, C# 9+ record, or use a custom Capture matcher to inspect/copy state at the time of invocation.