Last week, Microsoft released a new version, build 10051, of its Windows 10 Technical Preview for phones, and unlike the first preview, this one was opened up to a much wider range of devices - almost the entire Lumia Windows Phone 8.x range, in fact.
There were some known issues with the new preview when Microsoft pushed it out for those on the Windows Insider Fast track, but the company couldn't anticipate every possible problem that might arise - that is, after all, what these technical previews are for.
Indeed, it seems that build 10051 has been causing some unexpected issues for owners of the Lumia 520, Lumia 525 and Lumia 526 (a localized Chinese variant of the 525). Microsoft has acknowledged these issues in a post on its Community forums:
We are seeing some reports of failures on Lumia 520/525/526 devices when trying to roll back to Windows Phone 8.1 using the Windows Phone Recovery Tool.
The post went on to say that, as a result of these reported failures, Microsoft is "pausing build availability for these devices temporarily while we investigate the issue and work with customers who have reported problems." Several users on the company's forums reported that their devices had been bricked while attempting to roll back, and Microsoft is clearly keen to limit the impact of the issue for other users.
If you were planning to install build 10051 on one of these affected handsets, you'll have to wait until Microsoft can fix the issue. If you already have the build installed on one of these devices, you should probably avoid attempting to roll back to Windows Phone 8.1 anytime soon.