Invalid postback or callback argument – AJAX Control Toolkit

I was working to add some Ajax interactivity to a page that needed some asynchronous workings. Unfortunately, after displaying a ModalPopupExtender and the user clicking the Close button on the “popup”, the page would throw the following exception:

System.ArgumentException occurred
  Message=”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.”
       at System.Web.UI.ClientScriptManager.ValidateEvent(String uniqueId, String argument)

There are lots of explanations out there for this kind of problem, but none of the common answers really fit. The most common answer was to disable event validation. That wasn’t a good answer for me, I don’t think disabling an important security feature because you have complex code is a good solution. The other common answer was to override the render event to register the control posting back with the ScriptManager. This did not work at all for me.

After finding an reading through this post and the comments, I realized the problem was indeed in the code. After the call to ModalPopupExtender.Show(), the original code was re-binding a grid on the page. This order of events was causing the problem. I changed the binding to occur before the call to the ModalPopupExtender by changing which event containing the rebinding call, and the error went away.