Apache's HTTP Basic Authentication could do with more logging
Suppose, not entirely hypothetically, that you use Apache and have an area of your website protected with Apache's HTTP Basic Authentication. A user comes to you with a problem report; while interacting with this area of the site, they unexpectedly got re-challenged for authentication. In fact, in your Apache logs you can see that they made an authenticated request that returned a HTTP redirect and literally moments later their browser's GET of the redirection target was met with a HTTP 401 response, indicating that Apache didn't think they were authenticated or maybe authorized. Unfortunately, our options for understanding exactly what happened are limited, because Apache doesn't really do logging about the Basic Authentication process.
There is one useful (or even critical) piece of information that Apache does log in the standard log format, and that is whether or not the HTTP 401 was because of a lack of authorization. Both normally get HTTP 401 responses (although you can change that with AuthzSendForbiddenOnFailure and perhaps should), but they appear differently in the normal access log. If there was a successful authentication but the user was not authorized, you will see their name in the log file:
192.168.1.1 - theuser [...] "GET /restricted HTTP/1.1" 401 ....
If they are not authenticated (for whatever reason), then there will be no user name logged; the leading bit will just be '192.168.1.1 - -'.
However, there are at least five reasons why this request was not authenticated (in Apache's view) and you can't tell them apart. The five reasons are the browser didn't send an Authorization header, the header or a part of it was malformed, the authorization source (such as your htpasswd file) was missing or unreadable, the header contained a user name that Apache didn't find or recognize, or the password in the header was incorrect. It would be nice to know which one of these had happened, because they lead to quite different causes and fixes.
(Apache may log errors if the authorization source is missing or unreadable; I haven't tested. That still leaves the other cases.)
For example, if your logs say that the browser didn't send the header at all, that is probably not a problem on your side. Although the rules for when browsers decide to send this header are a bit complex and potentially surprising, because it doesn't work like cookies, where a browser will always send them once set. And browsers make their own decisions about how to react to HTTP 401 responses on requests where they didn't send Authorization headers, so they may decide to re-ask the person for a name and password even though they have Basic Authentication credentials they could try.
(Having discovered AuthzSendForbiddenOnFailure, I am probably going to set it on several limited-access areas in our Apache configuration, because it's rather more user friendly. It's not an information disclosure for us because there are authenticated but otherwise unrestricted areas on the web server with the same credentials, so an attacker can already validate guessed passwords.)