The domain name portion of a URL is not case sensitive since DNS ignores case:
http://en.example.org/
and HTTP://EN.EXAMPLE.ORG/
both open the same page.
The path is used to specify and perhaps find the resource requested. It is case-sensitive, though it may be treated as case-insensitive by some servers, especially those based on Microsoft Windows.
If the server is case sensitive and http://en.example.org/wiki/URL
is correct, then http://en.example.org/WIKI/URL
or http://en.example.org/wiki/url
will display an HTTP 404 error page, unless these URLs point to valid resources themselves.