From File->Backup History for FileX.c
Open an old version of the file.
Leave history.
You now have a buffer that is "Version XX of C:\....FileX.c"
Use the "Files" tool window and double click that file.
You get a status line of "Invalid buffer id" - and you cannot switch to that buffer :-(
Weird -- From "Files" I tried to close this file - didn't work.
But then I tried again to switch to this buffer, and it works now!