A Windows 7 PC updated itself to Windows 10. Not my PC, a client's. I didn't think it was supposed to do that.
A certain important program doesn't run in Windows 10. Probably it can be made to run, but that will take some work.
So I restored the machine to Windows 7. Which seemed to be straightforward and relatively successful. The important program ran again.
But now the PC is complaining that it doesn't have a legitimate copy of Windows 7. It does, mind you. It's just suffering from some kind amnesia.
Microsoft is being delusively protective,
and this update process was sadly defective.