A unit test that will periodically fail as the current day changes. From: http://hjerpbakk.com/blog/2013/9/24/the-little-test-that-could-not.html

BadTest.cs

[TestFixture]
public class BadUnitTestExample {
    [Test]
    public void AgeMustReturnCorrectAge_Bad() {
        var aPerson = new PersonBad { TimeOfBirth = new DateTime(1983, 9, 25) };

        Assert.AreEqual(29, aPerson.Age);
    }
}

public class PersonBad {
    public DateTime TimeOfBirth { get; set; }

    public int Age { get {
        var today = DateTime.Today;
        int age = today.Year - TimeOfBirth.Year;
        if (TimeOfBirth > today.AddYears(-age)) --age;
        return age;
    }
}