Invalid Callback Or Postback Argument

I wrote an ASP.NET 2.0 web page, which populates a drop down list according to the user input values. I put the drop down list into an AJAX UpdatePanel, which is triggered by some server controls. But when I try to run it, the following error prompts out:

Invalid postback or callback argument. Event validation is enabled using <pages enableeventvalidation="true" /> in configuration or <%@dotnet.itags.org. page enableeventvalidation="true" %> in a page. For security purposes, this feature verifies that arguments to postback or callback events originate from the server control that originally rendered them. If the data is valid and expected, use the ClientScriptManager.RegisterForEventValidation method in order to register the postback or callback data for validation.

Anybody knows how to solve it?

hello.

can you build a small demo page that reproduces this problem and put it here?

thanks.