HTTP 401 Unauthorized or 403 Forbidden for a "disabled" user?
An authentication service allows user accounts be disabled (a sort of soft-delete).
If the server then receives an authentication request for a disabled user that would otherwise be valid, should the server return 401 or 403? With either status code, I would return a message indicating that the account had been disabled.
For quick reference, relevant quotes from HTTP/1.1 spec (emphasis mine):
401 Unauthorized
The request requires user authentication. The response MUST include a WWW-Authenticate header field (section 14.47) containing a challenge applicable to the requested resource. The client MAY repeat the request with a suitable Authorization header field (section 14.8). If the request already included Authorization credentials, then the 401 response indicates that authorization has been refused for those credentials. If the 401 response contains the same challenge as the prior response, and the user agent has already attempted authentication at least once, then the user SHOULD be presented the entity that was given in the response, since that entity might include relevant diagnostic information. HTTP access authentication is explained in "HTTP Authentication: Basic and Digest Access Authentication" [43].
403 Forbidden
The server understood the request, but is refusing to fulfill it. Authorization will not help and the request SHOULD NOT be repeated. If the request method was not HEAD and the server wishes to make public why the request has not been fulfilled, it SHOULD describe the reason for the refusal in the entity. If the server does not wish to make this information available to the client, the status code 404 (Not Found) can be used instead.
Answer
Based on an email written by Roy T. Fielding, there's apparently a bug in the current HTTP spec.
The way the spec is intended to be read is as follows (using quotes from above email):
401 "Unauthenticated":
you can't do this because you haven't authenticated
403 "Unauthorized":
user agent sent valid credentials but doesn't have access
So, in the case of a disabled user, 403 is the correct response (and 404 is also an option).