This is probably a very obscure situation, but it happened to me, it could happen to someone else. Scenario: an ASP.NET page with Infragistics UltraWebGrid inside of a WARP panel. A button outside the WARP serves as a trigger for partial postback. First click on the button causes expected partial postback, but on the second click page does full postback and is screwed after that. The issue happens only in IE6/7, page works correctly in IE8.
Another condition – page contains ASP.NET AJAX ScriptManager control with ServiceReference path pointing to an ASMX WebService.
Turned out the issue was caused by project being left in debug mode (in web.config debug=”true”). Which caused WebService page to be loaded with parameter “jsdebug” in query string. Which apparently IE6 and 7 didn’t like very much. Switching to debug=”false” in web.config solved the problem.