I'm noticing some really weird behavior with the classifieds plugin since I started using it. In another post, I mentioned how the CSS doesn't work for the classifieds page if you change the values through the Theme Editor. Now, I've noticed (on a clean installation) that the http://yourdomain.com/forums link no longer works after the plugin is installed.
Steps to reproduce the problem:
1. Install Wordpress
2. Install Buddypress plugin (don't check the forum checkbox in settings)
3. Install BBPress plugin
4. Create a forum and visit http://yourdomain.com/forum and see that it works
5. Install Classifieds Plugin
6. Visit http://yourdomain.com/forums