title: System.InvalidOperationException: This document already has a 'DocumentElement' node
body: <p>I have a trouble on appending new node to xmldocument (created in the memory). I have select the root node with the XmlDocument.SelectSingleNode() method, it work sometimes and in the other time it will give me "System.InvalidOperationException: This document already has a 'DocumentElement' node." error. More information, this xml document is multi level xml document.</p>
  
  <p>By the way, when i try it with unit test it work fine (always), when i implement it in ASP.NET 3.5, it become weird, work sometimes and fail sometimes. Any idea, why this can help? All advise and suggestion are welcome.</p>
  
  <p>Thanks.</p>