But you can do the same mistake on Facebook, if I post a story to my wall and you comment on it, then you don't really know who gets to see it. The default sharing mode is "Friends of friends" if I remember it right, and that means my friends of friends can read your comment, and you can only see who my friends are, but not who my friends of friends are.
On the other hand, I think most people are used to the fact that posting on Facebook is pretty public, and that the Circles feature might give people a false sense of privacy so they are more likely to make that mistake.