"Invalid postback or callback argument" Problem
In case you get the following exception while trying to run an ASP.NET application, the solution is not as simple as setting the "enableEventValidation" configuration to "false" all the time.
Invalid postback or callback argument. Event validation is enabled using <pages enableEventValidation="true"/> in configuration or <%@ 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.
I had such a problem last week. One of my colleagues asked me to check a problem because he could not see it and everything used to work perfectly before. I must confess that I spent hours to find the problem. Even if I set the "enableEventValidation" value to "false", nothing worked and everything started to lose its ViewState data.
Finally, I’ve found that the problem was a forgotten "form" tag within a user control. So, if you have such a problem about losing ViewState unexpectedly and exceptions about Event Validation, double check your code not to have such <form> tags.