Some at Microsoft want the company to move on from its flailing mobile effort, but I didn’t think they’d take such drastic measures to get people off the platform. Insiders who downloaded build 16212 have come to find that their device is now in a boot loop, requiring reset using the Windows Device Recovery Tool, which wipes all files.
Microsoft accidentally released a new build of Windows 10 to Windows Insiders that you should probably avoid. The company released internal build 16212 for some PCs and Mobile devices to Windows Insiders. Build 16212 causes Windows 10 Mobile devices to enter a boot loop, requiring users to reset their device using the Windows Device Recovery Tool. Resetting your device will mean that you’ll have to get rid of all the files on your devices, and that’s something none of us probably want to do.
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Source: [H]ardOCP – Microsoft Releases Windows 10 Mobile Build That Bricks Devices