After upgrading to Mountain Lion, I get the following error message when I launch Calendar.app:
"Calendar Cache couldn't be moved because you don't have permission to access "Calendars".
When I click "OK", Calendar quits.
This user is an administrator on the machine. I tried granting the user 'Read & Write' access to Calendar.app, but nothing changed.
I have another administrator account on the same machine that can launch Calendar.app without issue.