Wikimedia's copy of MediaWiki has been updated to r51904 (r51864 for extensions). There were lots of fun bugs, the most exciting of which are now fixed.
Developers, please check the bug tracker for bugs reported against code that you maintain, or bugs that you can otherwise fix. Users, please report any bugs you notice there, after doing a search for duplicates, even if you also report them in other places such as IRC or [[WP:VPT]].
https://bugzilla.wikimedia.org/
Please do not discourage users from reporting "known issues", point them to the bug report so that they can give us a priority hint in a comment. It's hard to prioritise bug fixes when we don't know how many people are affected or how severe the issue is.
-- Tim Starling