What happened to me was, I bought a decent domain the other day for a new site I'm working on and was surprised to find the domain listed on about 10 other sites already. I was just checking to see if any engines had picked up my site yet, they hadn't, but I did find all the old references of the URL.
In my case it's totally fine, but could be a much bigger surprise for someone that is setting up a site for thier kids, or school, or something like that and then find their URL on other unrelated or worse inappropriate sites.