How can I diagnose or fix the CalendarAgent occupying a whole CPU on Mountain Lion?
May have to kill and rebuild Calendar db:


Disable the Calendar service from your account in System Preferences → Internet Accounts
Remove the preferences/database files for Calendar:

rm -r ~/Library/Calendars/ ~/Library/Preferences/com.apple.iCal.plist

Re-enable the service.


Source: http://robert.accettura.com/blog/2012/08/19/how-to-stop-calendaragent-from-eating-cpu/