Trying to help someone on IRC today.. their existing .htaccess file was very scary. It used the <Limit> directive, in completely the wrong context:
Quote from from a .htaccess file:
<Limit GET POST> require valid-user </limit>
This means that any other HTTP method could access the site without any authentication. The kicker is that mod_php will allow any HTTP method. It doesn't just restrict to GET or POST requests.
Apache by itself only by default allows GET and HEAD requests on static files. Anyways, let this be a warning, when you think you need <Limit>, you probally don't. Ever. Really. I am Serious. Do not use 'Limit'.
Most people really want <LimitExcept>. But, I believe that in most cases, you should avoid optional authentication. I think that authentication should be required for an entire path, and not optional, depending on how the client munges the request. Optional Client Certs are also a bastard to get working too, so the best solution is to just avoid the entire 'optional' authentication mindset.