Drupal support forum
So here's an example. Let's say you're running a site with forums. This is the support forum for Drupal, a content management system. People post their questions, and other people can help them out with answers.
Note: This is part of my presentation on Visual Security Policy for the Web.
Note 2: As far as i know, there is no such attack possible on the Drupal forums; I just used them because they made a nice screenshot.