Dragging keywords from the Keyword HUD to the Control Bar can result in inadvertent changes to the Keyword hierarchy in the HUD.
For example: You start to drag a key word from the Keyword HUD to the Control Bar, but you change your mind before dropping it onto the Control Bar. The keyword ends up in a different position within the HUD.
This happens if the mouse button is released while the keyword is over a new position. Aperture will interpret this as a request to reorder the keywords within the HUD. If you start to drag a keyword and then change your mind, always drag the keyword back to its original position.
In Aperture 1.5, you can prevent this from happening by clicking the lock icon at the lower left corner of the Keyword HUD.