Postback error intermittent
<blockquote>
  <p>Invalid postback or callback argument.  Event validation is enabled using enableEventValidation="true"/> in configuration or &lt;%@ 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.</p>
</blockquote>

<p>I have added the Page attribute and it still occurs from time to time - mainly when I have left the page open and click my update button that does some math with some DB variables located in controls. I am using a Session variable to store a client id so my entity is filtering based on the client's id. Any help as to where to look for fixing this would be great. The error message:</p>

<blockquote>
  <p>[ArgumentException: Invalid postback or callback argument.  Event validation is enabled >using  in configuration or &lt;%@ 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.]
    System.Web.UI.ClientScriptManager.ValidateEvent(String uniqueId, String argument) +144
    System.Web.UI.Control.ValidateEvent(String uniqueID, String eventArgument) +111
    System.Web.UI.WebControls.TextBox.LoadPostData(String postDataKey, NameValueCollection postCollection) +26
  System.Web.UI.WebControls.TextBox.System.Web.UI.IPostBackDataHandler.LoadPostData(String postDataKey, NameValueCollection postCollection) +13
    System.Web.UI.Page.ProcessPostData(NameValueCollection postData, Boolean fBeforeLoad) >+303
    System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +1960</p>
</blockquote>
