Some recent database changes may affect queries using the Quarry tool. Queries for site_stats at English Wikipedia, Commons, and Wikidata will need to be updated. Read more.
A new user_global_editcount variable can be used in abuse filters to avoid affecting globally active users. 
Changes later this week
The new version of MediaWiki will be on test wikis and MediaWiki.org from 21 June. It will be on non-Wikipedia wikis and some Wikipedias from 22 June. It will be on all wikis from 23 June (calendar).
Users of non-responsive skins (e.g. MonoBook or Vector) on mobile devices may notice a slight change in the default zoom level. This is intended to optimize zooming and ensure all interface elements are present on the page (for example the table of contents on Vector 2022). In the unlikely event this causes any problems with how you use the site, we'd love to understand better, please ping Jon (WMF) to any on-wiki conversations. 
Parsoid's HTML output will soon stop annotating file links with different typeof attribute values, and instead use mw:File for all types. Tool authors should adjust any code that expects: mw:Image, mw:Audio, or mw:Video.