We have a PHP web application that posts data to itself, and either displays an updated page based on that data, or redirects to another page. An example of this is a script with a paged list on it, where clicking on the Next link causes a post to the same page, which then returns an updated version of the page showing the new set of list items.

One client is reporting that IE is displaying friendly error messages when the page updates itself instead of the correct behavior of displaying the updated page.

Turning friendly error messages off "corrects" this problem, and displays the updated page normally, indicating no actual server error occurred.

When testing from any location other than this client's our web app does not produce any http error statuses, and in this specific situation only produces 200 statuses. (According to Fiddler.)

What could be interfering with the HTTP POST and changing the response's http status code to 500 (or another code that would trigger friendly errors in IE)? Are there certain proxies or other network tools that could be misconfigured or buggy in this manner?

Is there any way we can alter our application (apart from avoiding posts to the same script, which is not feasible) to get around this misbehavior?