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):
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
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.