Make sure private data is, in fact, marked private on Google Calendar.

Brian Krebs, a security expert at the Washington Post, has found that Google Calendar can be a useful source of info (logon names, passwords, phone numbers, addresses, hacks, cracks...) that shouldn't be publicly available:
It's a great Web-based tool, but in experimenting with it I found that far too many people are using Google Calendar without fully understanding how to protect their personal information. ...


By default, Google does not share your calendar entries with the rest of the Internet unless you actively choose to make your calendar public. The examples above generally arise from people who have chosen to share their calendars but neglected to make certain events private, or to select a certain few individuals who are permitted to have access to the entries.