When a space isn’t a space

I ran into a scenario recently where this test failed:

[Test]
public void You_have_to_be_kidding_me()
{
    string a = "You have to be kidding me       ";
    string b = "You have to be kidding me       ";

    a.ShouldEqual(b);
}

It took me quite a while to determine the problem, as it originally came from a column in the database with natural keys.

When a test like this fails, it’s time to call it a day.  Any guesses as to the problem?