Microsoft has apologized after mistakenly sending out preview versions of Windows 10 that weren’t ready and could cause problems. In some cases they even went to people who aren’t in the Insider test program.

The company says the blunder happened because of “an inadvertent deployment to the engineering system that controls which builds/which rings to push out to insiders.”

The build in question, 16212, went out to people regardless of which of the four rings (release schedules) of the program users were in.

It quickly blocked the updates from going out to anyone else and has halted all new build releases until next week. Microsoft isn’t offering precise figures but says “a small portion of folks got these builds.”

Windows 10 desktop owners who are in the Insider program are warned the new build “may include issues that impact usability of your PC – more so than the normal builds we give you.” Microsoft says those users can either wait it out for the next planned build, or roll back to a previous build.

The news is worse for Windows 10 Mobile users on the Insider program. Those who installed the mistakenly released build will find their device in a reboot loop. They’ll need to re-flash their device using the Windows Device Recover Tool, which will install the latest public release. Those users can then rejoin the Insider program, assuming they haven’t been put off.

Mobile users who got the mistakenly released build but haven’t installed it should backup their device, then reset it, which should remove the unwanted download. After resetting, they should check they are still listed on the right ring in the Insider program.

Users who aren’t on the Insider program at all may still have seen the rogue build downloaded, but Microsoft says it simply won’t install on those user’s devices so there’s no need to do anything.