Problem

I use Fantastical on Mountain Lion, which grabs data from Google Calendars. I’ve been having an issue happening more and more lately where I add an event via Fantastical, then add an invitee. Later, Calendar opens up with an alert:

The server responded with “400” to operation CalDAVWriteEntityQueueableOperation.

A few Google searches later, I finally found a workaround.

Solution

Source: discussions.apple.com

I went to Calendar preferences and switched to my Google account and changed all alert types to anything but “None”. Closed the window, and it seemed to work.