when using the /wiki/<pagename> URL format, adding parameters (such as ?redirect=no) fails when the pagename includes special characters, like '?'. This occurs even when the special character is correctly URL-encoded. For example:
https://en.wikipedia.org/wiki/Statistical_test?redirect=no does *not* follow the redirect to https://en.wikipedia.org/wiki/Statistical_hypothesis_testing; it remains at the Statistical test redirect, which is again expected.
However, https://en.wikipedia.org/wiki/%3f?redirect=no *does* follow the redirect to https://en.wikipedia.org/wiki/Question_mark, which is *not* expected. This is despite the fact that the first question mark is correctly URL-encoded. The final URL that we end up at appears in the browser bar as https://en.wikipedia.org/w/index.php?title=Question_mark&%3Fredirect=no.
Similarly, https://en.wikipedia.org/wiki/User:Writ_Keeper/sandbox%3Fsandbox?redirect=no also incorrectly follows the redirect to User:Writ_Keeper/sandbox; the final URL there ends up as https://en.wikipedia.org/w/index.php?title=User:Writ_Keeper/sandbox&sandbox%3Fredirect=no
It would seem that the URL is being decoded before the URL parameters are deciphered.