You can disable the shared calendar notifications in Notification Center on OS X. Although, the setting to turn it off isn't where you'd expect it to be.
Sometimes synchronization errors can result in iCal calendars and events multiplying. There are several ways to reset synchronization data, but here is a thorough approach to ensure that the synchronization system starts from a blank slate.
Hate having your Facebook events separate from your regular calendar? On today's Tekzilla Daily, Veronica shows you how easy it is to export them to Google Calendar, iCal, or Outlook.
iCal 1.5.4: iSync issues; iCal event problems
One problem you may run into when using iCal is the program constantly popping up with an error stating "Access to account 'account name' is not permitted." This may happen after syncing from another system, or after migrating to a new account or computer, but upon dismissing the error it may pop up again immediately or whenever the calendars are refreshed.
iCal 1.5.5: Problems with installation
Mail.app and iCal: The Y2K3 bug?
Apple releases iCal 1.5.5
For some users the iCal search function suddenly will not work. While they can enter search information in the search box, the results window shows no items, even though events exist that contain the search query.
iCal event bug confirmed