A few years back I ran a small forum. I didn't keep much of an eye on it. For about 3 days I didn't bother checking the site out, and when I did check it over, I saw someone had gotten in and deleted all of the forum topics/posts and topic areas. He replaced the background with an image of "YOU'VE BEEN HACKED" and that was it. I had a backup and fixed things, but it was a pain to deal with. I sold the forum off soon after.