title: Why does DateTime.ToString("h") cause exception?
body: <p>Open a watch window</p>
  
  <pre><code>new DateTime(2010,01,01).ToString("h")
  </code></pre>
  
  <p>Gives: </p>
  
  <blockquote>
    <p><code>new DateTime(2010,01,01).ToString("h")</code> threw an exception of type <code>System.FormatException</code></p>
  </blockquote>
  
  <p>Yet...</p>
  
  <pre><code>new DateTime(2010,01,01).ToString("h ")
  </code></pre>
  
  <p>Gives: "12 "</p>
  
  <p>So why does an extra space stop this format exception from happening?
  Is this a bug?</p>