Outlook 2003 - New appointment conflicts with phantom appointment

Just recently - when I create a new appointment in Outlook 2003 (with an appointment date july 1 08 or later), it automatically generates the yellow banner "conflicts with another appointment on your calendar."

Why?

There is nothing there that conflicts. And it affects every day of the week out into 2009.

I do sync with a Palm T|X, and with plaxo online. I've not added any appointments after July 1 from either of these platforms (very rarely add with Plaxo anyway.)