Microsoft has explained what caused it to accidentally release an internal preview build of Windows 10 to Insiders. And it has explained how one might roll back to a previous build.

As you may recall, Microsoft accidentally issued one of its internal daily builds of Windows 10 to all rings of the Windows Insider Preview yesterday. This was a 20H1 build, but it wasn’t intended for public consumption: Indeed, as Insiders started dutifully installing the build, they uncovered a secret Microsoft had meant to conceal: It includes the Live Tile-less version of the Start menu that the firm is developing for Lite OS, its coming Chrome OS-like system.

Today, Microsoft is confirming that it was all just a mistake.

“An internal preview build was unintentionally released to a broader audience than expected,” Microsoft’s Jason Howard explains in a post to the Microsoft Answers support site.

Better still, he explained what happened, a nice turnaround for an organization that isn’t known for its transparency.

“We actively test and validate each of our preview builds internally before sharing them externally,” he explains. “However a configuration change allowed this build to release to multiple rings simultaneously, including external rings.  We have made the necessary remediation in our Flighting service and have implemented additional changes to help prevent [this] from this happening in the future. We apologize for any troubles this may have caused for affected Insiders and appreciate your assistance in bringing this to our attention so we could make the necessary changes as quickly as possible.”

The bigger issue, of course, is that some Insiders did install this build. And while it’s fascinating on some level to see what’s happening in the future with the Start menu, most will want to restore their previous build. Otherwise, they may not be able to get back on the normal build delivery schedule, according to which ring their PC is in.

Howard provides detailed instructions on how to do this in his post. But the important point here is to move quickly, though I suspect that Microsoft will now delay further Insider builds until next week.