406 Not Acceptable

Hi all,

RFC 2616 says about "406 Not Acceptable":

"""
Note: HTTP/1.1 servers are allowed to return responses which are not
acceptable according to the accept headers sent in the request. In some
cases, this may even be preferable to sending a 406 response. User agents
are encouraged to inspect the headers of an incoming response to determine
if it is acceptable.

If the response could be unacceptable, a user agent SHOULD temporarily
stop receipt of more data and query the user for a decision on further
actions.
"""

This statements is quite unprecise to the least. From past experience what
are those "some cases" which may be preferable? How the UA should query
the server for further actions?

Regards,
- Sylvain