Have you ever had a friend send you a link via Instant Messenger that looks similar to this?
http://www.geocities.com/`some_user_name`/`some_filename`.html
This link actually works, you can visit it, but whatever you do, DO NOT ATTEMPT TO LOG IN!!!
Here’s a quick rundown of the scenario. You get the link, figure that “Hey, friend sent it, so it must be legitimate.” You visit it, oh it’s a photo album, fine you need to log-in to view it. Luckily you’re a Yahoo subscriber so you don’t need to register. You log-in… nothing happens, you’re redirected to the same page. You figure it must be a server error and just dismiss the issue… only to find out that you too are sending the same messages without your knowledge.