Update: Reports suggest that Microsoft has plugged the Insider app. Using Fiddler to spoof Windows 10 on your unsupported device will no longer work. Although this was only one method to install Windows 10 unofficially, it was a popular one for those without an SD card.

If you're running the Windows 10 preview for phones on your device, an update for the Windows Insider app is waiting for you bringing it up to version 1.2.217.1. There's no telling what the update does, but it would be a good idea to go ahead and pull it down.