Invalid postback or callback argument

I recently took over an asp.net C# website.   I'm fairly new to .net.  This morning the app generated auto email for an unhandled except that was thrown.  The Exception error is: 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 contr ...