Despite the caveat that the latest Windows 10 Mobile build, 10572, should only be installable for those coming from Windows Phone 8.1, it appears that some users on earlier Insider builds have been seeing the build available as a pending download. Microsoft is aware of the issue, and has put a short-term fix into place. However, if you're stuck seeing 10572 as pending, Microsoft has provided the following fix:
- Reboot your device
- Open the Windows Insider app
- Re-select the WIP ring of your choice (WIP Fast for currently affected users)
- (You may be prompted to reboot again)
So, if you're sticking with your previous Insider build and have seen this issue, you should be able to resolve it with the above steps. Let us know in the comments if this works for you!