I had only 600-700 gb of data on my Mac. Installed OneDrive, did not know that I had configured my NAS to do 800gb of cloud backups to OneDrive. And Time Machine downloaded that data into its backup. Although I had configured OneDrive not to download that folder to my Mac. So time Machine screwed up in the first place.
Ah, ok, that's a nasty bug/scenario. I long for the good old days one could just rsync the internal drive and bless it and you have a bootable clone, or make compressed disk image. Those were the days.