You can find the full source code for this website in the Seam package in the directory /examples/wiki. It is licensed under the LGPL.
You really have to understand how Seam works to understand this message. You see this issue most often during development when using the hot deploy classloader. When Seam dumps and reloads the hot deployable classes, instances of those classes may still existing in an active long-running conversation. When you attempt to use that conversation, worlds collide. You just need to flush the session.
Here's what the exception actually means. When Seam looks for an existing instance of a component, it checks for a context variable equivalent to the component name. It's expecting to find an instance of the component. If it finds a value of a different type, then it throws this exception. The context variable was basically used for a different purpose, which is illegal (or you have the hot deployment classloader scenario described above).