Reoccuring events created or modified while using Outlook Web Access in Exchange 2003, might show strange effects in Outlook 2007 calendar.
Reoccuring evvents set in OWA show up OK in OWA, but dates in the future are removed when checking the calendar in Outlook 2007. Some attendees do get the correct change while others are removed. On the modified date is shown for some attendees, while reoccuring events are removed.
The reason why this is ocurring is explained in following blog by Ryan Morlok: