Microsoft today has reportedly accidentally pushed out Windows 10 Mobile build 14998 to some Windows 10 Mobile users on the Fast Ring,
It is currently unknown what features or improvements it adds, however, warnings have been issued from those who have installed it that the build is potentially unstable and could render your phone unable to upgrade to the next build without downgrading first back to Windows 10 Mobile/Windows Phone 8.1 (whichever one your phone shipped with).
It is also worth noting that the last publicly released Fast Ring build for Windows 10 PCs has been one of the most unstable so far, featuring crashes of the explorer user interface and numerous other bugs, casting further doubt on the usability of a build of dubious origin.
Adding further credence to the accidental nature of the build, it is now 2am in Redmond, Microsoft has typically been known to release insider builds during the daytime so they can get a handle on feedback, coupled with the lack of update notes in the feedback hub and the label of the build as “Localization for English”, we’re going to file this under accidental push until further notice.
Anybody installing the localization 14998 update, do not I repeat do not install it or the update system will be broken for ever on (cont)
— Gustave M. (@gus33000) January 12, 2017
While personally, I was not able to find and verify this update on my handset so I assume Microsoft may have already pulled it before I became aware of it, nevertheless if you do see the “Localization for English” update, do not install it.