This dialog drives me crazy on a regular basis. It makes .rtf almost unusable in Word 2007. Perhaps that's the point. Microsoft may have invented .rtf, but it's an open format, so they don't want anyone to use it.

One solution that will fix the problem for a while, though only for a while, is to save your .rtf document as a .docx document, close it, reopen the .docx, then save it as a .rtf document. Your new .rtf document will be fixed, i.e. you'll no longer be harrassed with the dialog whenever you save.

Occasionally, it will come back. I've never been able to work out what changes to the document make it come back.

Also be aware that saving as .docx might alter the format of the document. Ironic, really, that in harrassing us with a spurious warning that "Some custom properties that are stored in this document will be lost," Microsoft forces us to re-save the document in such a way that formatting in the document really is lost.

Nice.