This is actually very important, especially in RTS games! If you have a unit selected, and he's standing near the edge of a cliff, and you right-click off the edge (intersecting lower-elevation terrian), then the naive Astar algo would make the unit run away from your mouse cursor, because it would search the whole map for a valid way to get there --- however the player very likely wanted to get as close as possible to the edge, e.g. to attack an enemy he spotted.

(In other words, when pathing to different elevations, the the player almost always wants the unit to "run up as close as possible" to the edge, not try to find a valid path all the way around the map.)

It's actually a very tough/interesting problem, because it's probably impossible to "always do what the user intends in every situation" by merely looking at his mouseclicks.