Load a map with draggable directions enabled using the provided DirectionsRenderer to display a panel and let the renderer render a route. 
 
Then drag the polyline to change the route. After the route has been rendered again click on the destination marker B inside the panel. You will notice that this click has no effect.

This can also be seen in the official example

http://gmaps-samples-v3.googlecode.com/svn/trunk/draggable-directions/draggable-directions.html