I often get into debates when a group or department attempts to implement Agile, then does a half ass job, then complains about how Agile failed. One of the issues is exactly what Ross Pettit wrote about in his Agile Journal Article, Mythical Agile Shortcuts.
These myths that he outlines are only the tip of the ice berg. There are dozens of others that can sink, or stifle a project. So if you're on an Agile Project, learn these, and avoid them like the management anti-patterns they are.