One of my managers has a phantom calendar. When she is in the calendar view, it looks as if she has NO APPOINTMENTS. However, if she mouses over where an appointment SHOULD be, the popup with details about that appointment does show up.
Once, when I switched from Week view to Work Week view, we saw the little orange rectangles of her appointments for a split second, and then they all disappeared again.
She's running the Zimbra client in the latest Firefox on WindowsXP.