A real common problem with using the AJAX Control Toolkit revolves around the rendering of the controls. Taking an example from the ASP.NET Forums, let's say you have a Tab Control on your page. Everything looks ok, and then you run it. You notice that there is a weird space towards the bottom of one of your tabs. D'oh! What could be wrong? Well, in general, usually the cause of these types of problems are related to one of two issues, Quirks Mode or CSS conflicts. In the ASP.NET Forum example above, it was a Quirks Mode issue. More...