There are two important parts in a URL for web servers to consider, which have slightly different rules:
Subdomain: Everything after
://until the next
Path: Everything after and including that
#if there is one.
/mail/u/0/for my inbox. The
#inboxis not sent by the web browser to the server.
Subdomains have stricter rules on allowed characters and they are case-insensitive. Chromium will actually convert all alpha characters to lowercase before sending the request.
On commit--blog, each blogger gets a subdomain for their blog, and that subdomain comes from their username. Until this past Tuesday, everyone who I've convinced to sign up and try it out has had all-lowercase usernames. @pR0Ps finally bucked the trend and uncovered this issue.
The fix is simple: query the lowercased subdomain against lowercased usernames. Woo.
Wikipedia on URLs. There is a lot there. Always interesting to find out new things that seem very familiar.