In WP 2.7, it seems to be too easy to accidentally make a post "Private".
It has happened to me twice and I'm not sure where I clicked it (because I didn't mean to click it), either under Write New Post > Publish Module > Visibility settings, or under Quick Edit. I re-edit my posts a lot (like 50 times or more per post, adding info as I build it), so there are a lot of times I'm clicking Update Post in the Publish module area, or using Quick Edit (that cool new feature of 2.7), and both those areas have a Private checkbox or radiobutton.
I realized that the post wasn't showing up on the homepage anymore. That is how I realized it had gotten marked Private. Otherwise I would not have even noticed it, and the post would be effectively hidden from everyone.
There's a post about in on the Ideas forum as well,
Never had this problem before WP 2.7, and I'm usually pretty aware of what I'm clicking. I feel like this is selecting the checkbox when I have clicked somewhere near but outside the checkbox on the same line, or something.