Here's my code, which works perfectly well on vanilla IE, but fails on IETester:

  Init : function() {
    $(document).ready(function() {
      with (Loader) {
        if (typeof(G_vmlCanvasManager) != 'undefined') setTimeout(Make, 500);
        else Make();
      }    
    });
  }(),

It's a fragment of Loader class, which uses canvas element for ajax loading animation.
The browser chooses to run Make() method after specified timeout, but then fails
to create canvas element as vanilla IE does. I've tested it with greater timeout
values, up to 5000, but no joy. The problem occurs with all IE versions.

See http://code.google.com/p/explorercanvas/ for details of excanvas workaround.