Outlook error codes - they change on repetition of same error

[mark.lay]

This is wierd..

The error I'm trying to trap is when the start date for
an appointment is later than it's end date.

Doing this line of code:

mobjAppointmentItem.Save

Will throw a different error number every time it is
execute, making it impossible for me to trap in the event
associated with my custom functionality.

However, if i simply press 'save and close' on the
appointment dialogue, Outlook correctly traps this error
and shows a sensible error msgbox.

How does Outlook that? and how can I?

Thanks, Mark