If $wgEnableSidebarCache = true, it seems the other projects feature doesn't work correct. This is the case on zhwiki.

The sidebar is put into a shared cache for the entire wiki ("zhwiki:sidebar:zh"). Extensions can then use the SidebarBeforeOutput hook to modify both cached and uncached sidebar, but the SkinBuildSidebar (which wikibase uses) is not run for cached sidebar.

For most wmf wikis $wgEnableSidebarCache is set to false, but apparently only for wikis with the Collection extension (most wikis! but not zhwiki and various others) due to problems that extension has with sidebar cache.

Version: unspecified
Severity: major
Whiteboard: u=dev c=backend p=2 s=2014-08-26