by loren_d_c » Wed Dec 07, 2011 2:02 pm
Do you have ShopSite Pro? If so, are you using a custom Order API script (configured under Merchandising -> Order API) to process order information when the orders are placed? If you are, then this may happen if the Order API has to output content to the browser, in which case it has to output its own Content-Type headers, causing any subsequent ones to just be rendered as text.
-Loren