It could also be a character that does not exist in a particular font.

Carol,
Thanks for getting back to me.  This happens after a macro is recorded. I
have recorded a macro to run an invoice.  I noticed that after running the
invoice macro and I make any additional changes to the document this is when
a little square inserts itself into the document at the point where the
changes were made.  I do not see this at the time.  I save the document and
when it is e-mailed or I print the document is when I can see it.
Thank you,
Nancy