This is one of those little bits of trivia that's also a real gem if/when you ever need it, so wanted to post the info so folks could file it away for the day it hopefully saves them from going nuts. ;)
In my case I had updated my wife's laptop to 10 1803, ran disk cleanup to get rid of the old Windows install, & the folder c:\Windows.old was still there. In Windows Explorer I drove down several folders in Windows.old until I got to the last one, which Windows said it couldn't find when I tried to look inside the folder, or delete, rename, or move it.
What worked was opening a command prompt as admin, then typing the command rd /s "\\?\C:\Windows.old\..." where ... represents the rest of the path.
Turns out the "\\?\" part is a bit arcane -- it specifies an extended length path.
stackoverflow[.]com/questions/21194530/what-does-mean-when-prepended-to-a-file-path
And I found out about using it here:
social[.]technet[.]microsoft[.]com/Forums/ie/en-US/be3fe04d-e725-4199-828c-30eef70fcf95/unable-to-delete-folder-could-not-find-this-item?forum=winserver8gen