Small and regular steps help avoid random mistakes:)
Originally shared by John Skeats
New Google Tip: Don’t Share to Communities by Mistake
Always check who your post will be shared with before clicking the POST button. That is especially important with the new Google because of a change in how posts are pre-addressed. The audience for a post is at the top of the post as circled in the attached screenshot.
Google has always pre-addressed new posts to the same audience as the last post you shared, but the new Google changes added a new twist to that. The classic Google+ only kept track of who you shared your last non-community post with. The new Google, however, remembers community posts and will pre-address your next post to a community if the last post you shared was with that community.
If your post is not addressed to the audience you want, click on the audience to select the audience you want.
Relevant to the successor platform Google Currents