HTTP 406 Error
Some work I've been doing recently has involved debugging a Mambo installation. The website had developed the curious ability to block the editing of certain articles, but allowed other ones through. These blocked attempts to save articles were resulting in a '406 Not Acceptable' error.
According to the W3 specification, this means:
The resource identified by the request is only capable of generating response entities which have content characteristics not acceptable according to the accept headers sent in the request.
Say what?
After a lot of hair-pulling I tracked the problem