After unintentionally releasing Windows 10 build 16212 for phones and 32-bit PCs today, Microsoft published a blog post with some details as to why this occurred, and what to do if you installed it.
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Build 16212 RSS
After a fiasco earlier tonight, three new Windows 10 builds with the number 16212 have shown up on Windows phones. As was the case earlier, it is not recommended that you install these builds.
It's unclear whether or not Microsoft actually intended to roll it out, but Windows 10 Mobile devices on the Fast ring are currently receiving build 16212, which is higher than the one teased.