Steps to reproduce:
Go to https://en.wikipedia.org/wiki/Special:OAuth

Expected result:
Something informative about what OAuth is

Actual result:
Same is visiting https://en.wikipedia.org/wiki/Special:OAuth/FooBar, as in its interpreted as an invalid OAuth code (E002 - per mediawiki, The user has been sent to a non-existent subpage of Special:OAuth. Probably a typo in the code of the OAuth app.)

Getting started instructions:

• Set up MediaWiki core via vagrant: https://www.mediawiki.org/wiki/MediaWiki-Vagrant
• Set up Extension OAuth: https://www.mediawiki.org/wiki/Extension:OAuth#Installation
• Get the informative message reviewed by task subscribers and place it in the appropriate code file
• Submit a patch in Gerrit: https://www.mediawiki.org/wiki/Gerrit/Tutorial#Submit_a_patch