Outlook 2003 SP2, Exchange 2003

Have had this error several times now and don't know why we are getting it.
The scenario is an admin assistant goes into their manager's calendar and
adds an attendee to an existing meeting (not recurring). The manager is the
organizer of the meeting. When they click update they get the following
message:

"The item could not be saved because it has been changed by another person.
Do you want to save a copy in your default Calendar folder? If you click Yes,
you need to copy and paste your changes from the "copy of" item in your
default folder into a new update for this meeting."

As far as the admin assistant knows no one has made any changes to the
meeting. Is this error because they are not the organizer?

What really happens when they answer Yes to this error? Does the copy go to
the organizer's calendar or to the admin assistant's calendar?

Any ideas would be greatly appreciated.