When UpdatePanel is enabled, an ASP.NET webserver will send partial updates to the browser using AJAX postback. When this happens Test Studio is unable to detect dynamic targets. This causes the load test to fail because the load test will send the recorded value instead of the correct dynamic value. Here's an example:

|0|hiddenField|__LASTFOCUS||2780|hiddenField|__VIEWSTATE|27sKXhVmpoaAt72NcJfPUELFSPl7O8p1Gq07R1nq...

Attached internally is a .zip file containing a customer test and two fiddler traces demonstrating the problem.