Calendar reminder freezes and requires reboot to get rid of I have the Macbook Pro and use the Calendar program that came with it.  I have reminders come up an hour before my appointment but it does not allow me to "CLOSE" or "SNOOZE" it and when I put my cusor over it it shows the system bust WHEEL turning.  How can I correct this without losing all my calendar data?  I have to RESTART my system in order for the reminder screen to reset.  This started happening after one of the last system updates Mountain Lion OSX
Found a Fix, although it's not a definitive solution:

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I've been seeing this for some time now and getting quite sick of it. It happens when I've been away at work for the day and come home to find frozen notifications.

You can fix this temporarily by quitting Notification Centre from Activity Monitor but it keeps happening to me every week or so.
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