If you have to do a factory reset on your Windows Phone 8.1 handset or roll back a Windows 10 Mobile phone to the older OS, you might want to wait a little bit before proceeding. Some users who recently did one or the other are reporting that they can't sign in to their Microsoft account during the out-of-box experience or later through the phone's settings (via Neowin). The issue may not affect all Windows Phone 8.1 users at the moment, though Microsoft has yet to acknowledge the bug that recently surfaced on Microsoft's Community website.
The issue is a pretty serious one, as being unable to sign to your Microsoft account makes it impossible to download apps from the Windows Store (including Microsoft's Upgrade Advisor app, if you want to update your handset to Windows 10 Mobile) or sync your data stored on OneDrive, OneNote or Office Mobile. In other words, you may lose a lot of what makes your smarphone "smart" in the first place.
This is hopefully a temporary issue, and it seems that Windows 10 Mobile users are not affected as of today. But as 77.1% of the Windows Phone user base is still using Windows Phone 8.1 according to AdDuplex, we hope that Microsoft will be quick to release a fix. Let us know in the comments if you have been affected by this issue after resetting your phone.