https://arstechnica.com/information-technology/2018/06/facebook-bug-made-posts-by-14-million-users-readable-by-anyone/

Facebook disclosed a new privacy blunder on Thursday in a statement that said the site accidentally made the posts of 14 million users public even when they designated the posts to be shared with only a limited number of contacts.

The mixup was the result of a bug that automatically suggested posts be set to public, meaning the posts could be viewed by anyone, including people not logged on to Facebook. As a result, from May 18 to May 27, as many as 14 million users who intended posts to be available only to select individuals were, in fact, accessible to anyone on the Internet.