This has been reported twice on enwiki:
It appears that something is loading the 'sidebar' message incorrectly, somehow the intended value is being returned as element 0 of a two-element array with element 1 being a floating point number slightly less than 30. This, instead of getting the usual sidebar with boxes for "navigation", "search", "interaction", and "tools", each with the expected links inside, we get a sidebar with a box labeled "0" having broken subitems named "Navigation", "<SEARCH>", "Interaction", and "<TOOLBOX>" and a box labeled "1" with a floating point number inside.
rMW0a38dbc809c8: objectcache: Support key versioning in WANObjectCache looks suspicious, specifically the use of the return value of $this->unwrap() near line 897. See inline comments in Gerrit.