I think this is a major bug breaking Mediawikis since 1.26. This is a problem especially for non-English wikis containing special characters in article titles. It would be nice if this gets more attention.

Here's a pretty dirty hotfix you probably shouldn't use:

Short addition:
RFC 3986 says that the HEX chars are case-insensitive. A string comparison might not be the right thing to do here.

In T131414#2352017, @tstarling wrote:

In REQUEST_URI you have %c3 instead of %C3. Assuming your browser sent %C3, something must have decoded it to unicode and re-encoded it back to lower-case hex.