Overnight Microsoft accidently pushed out new PC and Mobile builds not intended for Windows Insiders. The builds weren’t intended for public testing and Microsoft managed to pull the builds before to many insiders picked up the update.
For PC there was a RS_EDGE_CASE release which Microsoft say if you did install you can either roll back or wait for a new build next week.
The situation is worse for mobile with 16212 being pushed out which causes the phone to get stuck in a boot loop. The only way to fix the device is to use the Windows Device Recovery Tool and re-flash Windows 10 Mobile. It’s a good illustration of why you should be careful of which devices you put on the Windows Insider Program.
It’s interesting to see that the build number 16212 is a lot higher than the current “feature 2” build currently public (15215). The label is RS_IOT which probably means it is designed for IOT devices.
You can read Microsoft’s details of the problem over on the Windows blog.