Earlier today, Microsoft released Windows 10 Mobile build 14998 to the Fast ring, although apparently it was not on purpose. It appeared in Windows Update as 'Localization for English' early in the morning.
Now, Dona Sarkar, head of the Windows Insider Program, tweeted that Microsoft is investigating the build and that "you don't need to install this one."
It's surely going to be tempting for Insiders to install 14998, as there hasn't been a new Insider Preview for phones in almost a month and a half, with 14977 arriving on December 1. In fact, there have even been two PC builds since then.
There are likely to be a number of new features in this build, but some users are reporting that they're having issues, such as a BSOD. Installing it may result in having to reset your device through the Windows Device Recovery Tool.