(2 secs) Consider this quite common way of adding class and instance methods to an class.
For most classes, this is perfectly fine, but it is not the most extensible way to add the methods to the class.
The problem with this way is that it is difficult for someone to override one of the class or instance methods you defined and call super to get the default behavior.
Yehuda touched on this near the end of his presentation yesterday, so I'll try to go into a little more detail.