Control registers its events during rendering and then validates the events during postback or callback handling. For example, if a list control includes options numbered 1, 2, or 3 when the page is rendered, and if a postback request is received specifying option number 4, ASP.NET raises an exception. All event-driven controls in ASP.NET use this feature by default.
Actually all that has to be done is to add this line either in the web.config file:
<pages validateRequest="false" enableEventValidation="false"></pages>
Or this line on any desired page:
<asp:Page EnableEventValidation="False" />