I won't repeat what they said here because it was pretty obscene and rather gross, but two friends of mine (they're married) have this running joke between each other where they send each other raunchy private messages all day on Facebook while they are at their desks at work. Then they each try to out-do each other with something even more vulgar and gross - just for shock value and laughs.
Anyhow, the one friend had just gotten her first Android smartphone, and apparently didn't quite understand how to use the Facebook app for it. Long story short, she thought she was sending these trashy messages to her husband as private messages, but she was actually posting them right on his wall for everyone to see.
I happened to notice it because they started showing up in my news feed, lol, and I put two and two together and realized what was happening, so I called her right away to warn her. But by then the damage was done, she had posted about two or three really embarrassing things on his wall. He ended up shutting down his Facebook account over the whole ordeal.