As reported by Jacob Christensen on pan-users, this is the scenario:

1. Select an article in the article headers pane.
2. Assuming I want to extend my selection down a few articles, I hold down
the SHIFT key and select another article a bit later in the list.  Pan
correctly extends selection from the original to the current item.
3. Still keeping the SHIFT key pressed, I change my mind and click on
another article in the opposite direction in the list away from the
original.  Pan correctly selects the articles from the original selection
to the new selection, removing any previously selections.

Incorrect behavior results when the SHIFT key is released, but then
depressed again while selecting an item to extend the selection.  In this
case Pan behaves as if I had pressed CTRL + SHIFT to "extend the selection
and include all preivious selections" in Windows UI behavior.

Pan should not do that.  Pan should only extend selection in one direction
in the list if the SHIFT is key is pressed and an article is selected.

Hope this is clear, otherwise I can explain better!