There doesn't seem to be an actual error occurring -- the Dismiss box simply isn't going away.  Here's one console log content that comes up when pressing the Dismiss button when it fails:

CalDAV: send: BEGIN:VCALENDAR
PRODID:-//Mozilla.org/NONSGML Mozilla Calendar V1.1//EN
VERSION:2.0
BEGIN:VTIMEZONE
TZID:America/Los_Angeles
X-LIC-LOCATION:America/Los_Angeles
BEGIN:DAYLIGHT
TZOFFSETFROM:-0800
TZOFFSETTO:-0700
TZNAME:PDT
DTSTART:19700308T020000
RRULE:FREQ=YEARLY;BYDAY=2SU;BYMONTH=3
END:DAYLIGHT
BEGIN:STANDARD
TZOFFSETFROM:-0700
TZOFFSETTO:-0800
TZNAME:PST
DTSTART:19701101T020000
RRULE:FREQ=YEARLY;BYDAY=1SU;BYMONTH=11
END:STANDARD
END:VTIMEZONE
BEGIN:VEVENT
CREATED:20130408T052223Z
LAST-MODIFIED:20130415T005334Z
DTSTAMP:20130415T005334Z
UID:92856606-55fc-443a-9810-e168dcf390a7
SUMMARY:Doctor Who
STATUS:CONFIRMED
RRULE:FREQ=WEEKLY
X-MOZ-LASTACK:20130415T005334Z
DTSTART;TZID=America/Los_Angeles:20130414T123000
DTEND;TZID=America/Los_Angeles:20130414T133000
X-LIC-ERROR:No value for LOCATION property. Removing entire property:
SEQUENCE:2
TRANSP:OPAQUE
X-MOZ-GENERATION:3
BEGIN:VALARM
ACTION:DISPLAY
TRIGGER;VALUE=DURATION:-PT15M
DESCRIPTION:This is an event reminder
END:VALARM
END:VEVENT
END:VCALENDAR

CalDAV: recv: 

CalDAV: Item modified successfully on Tom and Anthony Joint

CalDAV: send(https://www.google.com/calendar/dav/3k7fo7egba00ogijevme0452qc%40group.calendar.google.com/events/): <?xml version="1.0" encoding="UTF-8"?>
<C:calendar-multiget xmlns:D="DAV:" xmlns:C="urn:ietf:params:xml:ns:caldav"><D:prop><D:getetag/><C:calendar-data/></D:prop><D:href>/calendar/dav/3k7fo7egba00ogijevme0452qc%40group.calendar.google.com/events/92856606-55fc-443a-9810-e168dcf390a7.ics</D:href></C:calendar-multiget>

CalDAV: recv: 
<D:multistatus>
 <D:response>
  <D:href>/calendar/dav/3k7fo7egba00ogijevme0452qc%40group.calendar.google.com/events/92856606-55fc-443a-9810-e168dcf390a7.ics</D:href>
  <D:propstat>
   <D:status>HTTP/1.1 200 OK</D:status>
   <D:prop>
    <D:getetag>"63501657861"</D:getetag>
    <caldav:calendar-data>BEGIN:VCALENDAR
PRODID:-//Google Inc//Google Calendar 70.9054//EN
VERSION:2.0
CALSCALE:GREGORIAN
X-WR-CALNAME:Anthony and Tom Joint
X-WR-TIMEZONE:America/Los_Angeles
X-WR-CALDESC:For stuff Anthony wants to book w/Tom and Tom wants to book w/
 Anthony.\n
BEGIN:VTIMEZONE
TZID:America/Los_Angeles
X-LIC-LOCATION:America/Los_Angeles
BEGIN:DAYLIGHT
TZOFFSETFROM:-0800
TZOFFSETTO:-0700
TZNAME:PDT
DTSTART:19700308T020000
RRULE:FREQ=YEARLY;BYMONTH=3;BYDAY=2SU
END:DAYLIGHT
BEGIN:STANDARD
TZOFFSETFROM:-0700
TZOFFSETTO:-0800
TZNAME:PST
DTSTART:19701101T020000
RRULE:FREQ=YEARLY;BYMONTH=11;BYDAY=1SU
END:STANDARD
END:VTIMEZONE
BEGIN:VEVENT
DTSTART;TZID=America/Los_Angeles:20130414T123000
DTEND;TZID=America/Los_Angeles:20130414T133000
RRULE:FREQ=WEEKLY
DTSTAMP:20130414T212421Z
UID:92856606-55fc-443a-9810-e168dcf390a7
CREATED:20130408T052223Z
DESCRIPTION:
LAST-MODIFIED:20130414T212421Z
LOCATION:
SEQUENCE:2
STATUS:CONFIRMED
SUMMARY:Doctor Who
TRANSP:OPAQUE
X-LIC-ERROR;X-LIC-ERRORTYPE=VALUE-PARSE-ERROR:No value for LOCATION propert
 y. Removing entire property:
X-MOZ-GENERATION:2
BEGIN:VALARM
ACTION:DISPLAY
DESCRIPTION:This is an event reminder
TRIGGER:-P0DT0H15M0S
END:VALARM
END:VEVENT
END:VCALENDAR
</caldav:calendar-data>
   </D:prop>
  </D:propstat>
 </D:response>
</D:multistatus>

aChangeLogListener=undefined
calendarURI=https://www.google.com/calendar/dav/3k7fo7egba00ogijevme0452qc%40group.calendar.google.com/events/ 
iscached=true
this.mQueuedQueries.length=0

[calFilterProperties] Applying filter:
  start: 0
  end: 0
  status: 15
  due: 15
  category: null