 <p>Per the <a href="http://msdn.microsoft.com/en-us/library/h0hfz6fc.aspx" rel="nofollow noreferrer">MSDN documentation</a> the valid values for mode are:</p> <p><strong>On</strong>: <em>Specifies that custom errors are enabled. If no defaultRedirect attribute is specified, users see a generic error. The custom errors are shown to the remote clients and to the local host.</em> </p> <p><strong>Off</strong> <em>Specifies that custom errors are disabled. The detailed ASP.NET errors are shown to the remote clients and to the local host.</em> </p> <p><strong>RemoteOnly</strong> <em>Specifies that custom errors are shown only to the remote clients, and that ASP.NET errors are shown to the local host. This is the default value.</em> </p> <p>The default is RemoteOnly. </p>