Considerable editing issues presented themselves to users around the world for more than an hour on December 25 (wikitech-l). Between 18:50 and 20:20 UTC, edits were lost on a number of Wikimedia sites, although the problem was resolved cleanly on most, including the English Wikipedia. Only the Hebrew Wikipedia suffered lasting problems beyond the 90-minutes within which most problems were resolved on almost all sites. The glitch highlights the challenges of maintaining the stability of a website with as many visitors as Wikimedia sites over holiday periods when volunteers are few on the ground.
Another, unrelated issue, also came to light on Christmas Day. Bug #26429 ("Fatal error: PPFrame_DOM::expand") blocked a number of actions on the English Wikipedia, and quickly generated reports from a number of WMF wikis. Fortunately, the error only temporarily blocked editing and was worked around by reloading the page; it was reported as fixed on the afternoon of 26 December (UTC).
Not all fixes may have gone live to WMF sites at the time of writing; some may not be scheduled to go live for many weeks.
With the resolution of bug #26412,  links are no longer shown alongside the header on search pages, which are not editable by their very nature.
A new module has been added to the MediaWiki API. QueryPage; it allows users to generate the output of certain "query"-based special pages in a machine-readable format, rather than via HTML screen-scraping (bug #14869).