After encountering CAL-240 while using the calendar plugin on the as-yet-unreleased 2.8 candidate on extranet and getting the general feeling that there could be more bugs in it (it used to eat entire calendars for no clear reason when i used it last), I believe that the calendar plugin would benefit from being re-tested on the latest public versions. I've added a warning message to the extensions page until we're able to certify which versions it is still compatible with - see http://jira.atlassian.com/browse/CONF-11045