Until you're suddenly participating in a social network under the default (public) privacy setting without realizing or intending it.
As what happened when Buzz was (c)rudely integrated into the rest of Google+.
One minute you're sharing links and sending short messages to a small private group. The next, your posts are all publicly visible on a public profile page you didn't know existed.
I don't remember integrating Buzz into my Google+ page but I've never had issues over-sharing on Google+ - it's one of the things Google+ makes extremely simple.
As what happened when Buzz was (c)rudely integrated into the rest of Google+.
One minute you're sharing links and sending short messages to a small private group. The next, your posts are all publicly visible on a public profile page you didn't know existed.