Have you ever faced with UT that failed once and then you always see it succeeded?

One of such tests could be test where you verify that date properties of some newly created entity are equal if they take DateTime.Now for that.

Today Hudson showed me that some test failed in project I worked near half a year ago, so test looked like:

var item1 = new Item();
var item2 = new Item();
//…
Assert.AreEqual(item1.CreatedDate, item2.CreatedDate);

Test failed because time difference between dates was greater than few milliseconds. This could occur when processor is too occupied.

So I changed assert call to more realistic:

public void AssertDatesAreEqual(DateTime dateTimeLeft, DateTime dateTimeRight)
{
     Assert.IsTrue(Math.Abs((dateTimeLeft – dateTimeRight).Milliseconds) < 100,
    “TimeSpan difference between dates is greater than 100 milliseconds.”);
}