Not too long ago, Dona Sarkar teased the first new Windows 10 Fast ring builds in over two weeks, which would be 16203 for PCs and 15220 for Mobile. But that news was quickly overshadowed when phones on the Fast ring began receiving build 16212.1001.rs_iot.170531-1800.
The really interesting thing here is that this is the first 16xxx build for phones. Previously, a new 'feature2' branch had been created for Mobile devices, which hasn't yet shown any new features. It's been assumed that feature2 is a Windows Phone 7.8-style update that will mark the end of life for most existing devices.
But 16212 is from an entirely different branch. While it seems clear that this build wasn't released on purpose (after all, a feature2 build was teased), it should give us some insight into what's being developed on the Mobile side of the Redstone 3 Development Branch.
If you're on the Fast ring and you want this build, you should probably install it now, because it seems likely that Microsoft will be pulling this build at any moment. Note that if you do choose to install it, you're doing so at your own risk. We don't exactly know what happens after updating to 16212, and Microsoft hasn't said anything yet.
Update: According to @tfwboredom, the new build is not just rolling out to the Fast ring, but also Slow and Release Preview.
Update: This build is also rolling out to phones on the Production ring. Also, it's strongly recommended that no one install this build, no matter which ring you're in. Many users are finding that their devices are in a boot loop.
Apparently, 16212 rolled out to some desktop PCs as well. If you're among that group, it's recommended that you don't install that either.