I was coding a small proxy server. It worked quite nicely with most webpages. But it had problems with Wikipedia.
Firefox issued a request like this:
GET http://upload.wikimedia.org/wikipedia/commons/6/63/Wikipedia-logo.png HTTP/1.1 Host: upload.wikimedia.org
but Wikipedia responded with (trimmed down version):
404 Not Found The resource could not be found. Regexp failed to match URI: "http:/upload.wikimedia.org/wikipedia/commons/6/63/Wikipedia-logo.png"
However when I deactivated the proxy it worked. First I was puzzled but then I found out what the reason was. It was stated in the HTTP/1.1 Standard (http://www.w3.org/Protocols/rfc2616/rfc2616-sec5.html#sec5). In Section 5.1.2:
To allow for transition to absoluteURIs in all requests in future versions of HTTP, all HTTP/1.1 servers MUST accept the absoluteURI form in requests, even though HTTP/1.1 clients will only generate them in requests to proxies.
So this means that Firefox issues other requests if a proxy is configured (absolute URIs instead of relative ones) and that Wikipedia doesn’t obey the HTTP Standard as it should accept the absolute URIs as well