If you are using a control's SelectedIndexChanged event as a trigger for an UpdatePanel, you may be surprised to find that it only fires a callback when you change from the initial index, but not when you change back.  Long story short, you need to have the control inside of an UpdatePanel, so that when you change the index, the control can be changed to fire an event for the now-unselected item.  For more details, check out Steve Marx's explanation here.

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