I have a newly discovered problem with Calendar and iCloud. An organizer of some of the meetings I accepted told me that I sent him an "accepted" message twice, once when I accept the meeting and the second time 15 minutes before the meeting begins. The only thing that I do at that moment is closing the notification center message on my Mac. Why does this trigger a mail to the organizer? Sounds like a bug for me...
MacBook Pro (Retina, Mid 2012), OS X Mountain Lion (10.8.2)