Sometimes a column is not the atomic piece of data you thought it was. See Essay 10.1 for the first part of this rant, which address multi-dimensionality.

A second example of a column not being an atomic piece of data is when we have to support navigation through a tree (or graph) of information. In this case, you could represent the entire thing as a join table of parent and child pairs.

parent	child
top	child 1
top	child 2
child 1	child 1.1
chile 1	child 1.2

While correct, no one can easily navigate that structure in pure SQL. You either have to resort to proprietary extensions to the SQL, or hope that your visualization tool is up to the job. You can do a transitive closure on this structure and expand the columns to show multiple generations of parents and children. You can call this limiting -- it isn't first normal form. However, it's derived from the proper 1NF view and is merely an aid to navigation

level 0	level 1	level 2	level 3
top	Child 1	Child 1.1
top	Child 1	Child 1.2
top	Child 2	Child 2.1	Child 2.1.1
top	Child 2	Child 2.1	Child 2.1.2

This structure is what most people think of when the think of a hierarchy. The down side of this presentation is the hand-wringing that goes with maintenance. After all, they wouldn't be considering a data warehouse if they had this hierarchy problem solved. The transitive closure algorithm is painful to explain, so an example like this can help understand the mapping from a small, easily maintained, normalized piece of data to this, denormalized representation of the same data.