I have a client using shared calendars set up in Exchange (hosted by Rackspace.com) and viewed via Apple iCal.
This particular user is getting the following message:
iCal Can't Save the Event to the Exchange Server.
The account "email@example.com" currently can't be modified. To discard your changes and continue using the version of your calendars that's on the server, click Revert to Server. To save your changes on your computer until the problem is resolved click Go Offline."
We click on "Revert to Server" and message disappears. But then message pops up again a minute or two later.