I was working on a legacy page today that uses an AJAX POST and the response is sent to an IFRAME. The POST was successful, the results were valid and present in the IFRAME. The problem was as soon as the content of the IFRAME was eval'd() a GET was being sent to the server for the URL of the page. This GET was a problem because it had a side effect of clearing the session data (which contained the POSTed AJAX data) for the current page. So when the page's FORM was finally POSTed the server couldn't find the data in the session.
After trying numerous code changes including disabling event handlers and events, it finally occurred to me that the cache disable feature of the add-on might be causing the unexpected GET. Sure enough, once I re-enabled caching the GET stopped being sent.
It was strange that the eval() caused the GET to be requested. All the other code around processing the IFRAME's data didn't trigger it, only the eval. Strange but true.
So just something to keep in mind when strange behavior occurs on AJAX pages where session data is being used.