title: "Invalid postback or callback argument" after new deployment
body: <p>Recently deployed a project into production and have run into the "Invalid postback or callback argument" error. We haven't encountered this in testing at all and after some research have found that the problem occurs in the following situation:</p>
  
  <ol>
  <li>Old version is published and accessed.</li>
  <li>New version is published and accessed without clearing the Temporary Files.</li>
  <li>Drop down is changed twice. (The first time everything works fine.)</li>
  </ol>
  
  <p>The fix for the clients that have called in has been to clear their temporary internet files but this isn't the ideal fix. Can anyone think of a reason why this would be happening and stop happening after the temp files have been cleared?</p>
  
  <p>BTW: The app is ASP.NET 3.5 written in C#. We're using a javascript call back in this particular control that's causing this issue.</p>