You learn something every day. I was sure URLs are not case sensitive until now.
While domain names are not case-sensitive, the rest of the URL might be. In our example, this would be everything that follows “.com” as in wisegeek.com/are-urls-case-sensitive.htm.
Somewhat related, it took me an embarrassingly long time to realize that browsers treat assets with different URL casing as different assets for purposes of caching. Three image elements that reference Foo.jpg, foo,jpg, and foo.JPG all require a separate request and space in the cache, even if the web server you're using is case-insensitive and all three URLs resolve to the same image.
Ideally, it would do a redirect to the ‘canonical’ resource (all lowercase, e.g.), so that the browser only has to cache stuff once – but that would take another request at least once. Is there some way for the server to serve the content and redirect in the same reply? As in ‘Here’s your picture, but it is really called foo.jpg rather than FOO.jpg’?
While domain names are not case-sensitive, the rest of the URL might be. In our example, this would be everything that follows “.com” as in wisegeek.com/are-urls-case-sensitive.htm.