Certain patterns can be harmful or dangerous for your site.
Aliases that begin at the root
If you have an alias such as [title-raw] for a piece of content that users can create you allow them to create pages like "http://www.example.com/google1234567.html" that are used to authenticate Google or Yahoo! webmaster tools. This then allows the user to authenticate as the owner of the site and control the entries inside those tools (including removing your site from the index, changing between www and no-www versions of your site, telling the search bot to visit less frequently). This is not the default and is not recommended. If you want short aliases at a minimum make your pattern something like "c/[title-raw]" to put a character "c" before every title.
For example, certain situations where you create a pattern like forum/[title] and you have "Create Index Aliases" turned on pathauto will then create a page at example.com/forum/ which over-rides the default example.com/forum page. In order to fix this problem you can clear out that entry from the url aliases either via the GUI at example.com/admin/path or via the database table. A better path to create would be forums/ or discussions/ so that the names don't collide.