title: MongoDB difference between error code 11000 and 11001
body: <p>According to this <em>HIGHLY incomplete</em> list <a href="http://www.mongodb.org/about/contributors/error-codes/">http://www.mongodb.org/about/contributors/error-codes/</a> they're both related to duplicate keys. But I was not able to get a 11001 error. All of the following threw a 11000 error:</p>
  
  <ul>
  <li>inserting a document with an <code>_id</code> that already existed</li>
  <li>inserting a document with duplicate fields where the fields had a compound unique index</li>
  <li>updating a document with said compound unique index</li>
  </ul>
  
  <p>So this goes completely against the linked page, which says 11000 is for <code>_id</code> and 11001 would occur on updates (not inserts).</p>
  
  <p>So my question is: When does 11001 occur?</p>