ASP.NET controls create client-side script to raise post-back events on the server. Because a malicious user could use the postback script to send arbitrary post events to server controls, ASP.NET 2.0 controls validate the event data to ensure the event was raised by client-side code rendered by the control.
Setting the EnableEventValidation property to false returns the control behavior to the same as ASP.NET 1.0.
You can set the EnableEventValidation property for a Web application by setting the enableEventValidation attribute of the pages Element (ASP.NET Settings Schema) in the application's Web.config file, or for a single page by setting the EnableEventValidation attribute of the @ Page directive.