Calendar Events Disappear For Some but Not All Users

[eatickets]

I have a user who has a number of calendar events that have disappeared off her Main calendar. Many of these events had invitees who have shared their calendar with her which allows her to look back and confirm that the event is still on their calendar but not on hers. Also, the events still list her as part of the event (as either the admin of the event or one of the invitees), but she still can't see the events on her calendar.

She's accessing the calendar via the Zimbra webmail GUI, Apple iCal and a Microsoft Exchange account on her Droid. Any ideas?