Okay, I think I figure it out.  At least for my particular case.

The message "document not saved" was being triggered by a VBA code
module of a worksheet that contained lines of code that were too long
to fit on one line (>1024 characters).  These lines of code were being
put there by a macro, and of course the VBA editor doesn't like them.
Deleting the suspect lines of code immediately resulted in the file
being able to be saved.

I found this behavior to be 100% repeatable.

You can test this yourself by:
1. Start with a blank workbook
2. Right click on a sheet tab and select View Code
3. Paste in the lines shown below (the long one is about 1100
characters)
4. Try to save the workbook file.

I think the main point here is that Excel didn't like something in the
file, and so it would not save it.  So I would expect other things can
produce the same symptom.

Brian Murphy
Austin, Texas

Sub test()
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End Sub