Adam controls 1 Plains, 6 Forest, Garruk's Horde, and Selvala, Explorer Returned.
The top card of his library is Terastodon.

It is my understanding that per 601.2, he can cast Terastodon using Selvala's mana because it moves to the stack before he has to use Selvala's mana ability.

However, what happens if only lands are revealed, so Selvala does not produce mana? (Or you needed her to produce 2 mana, but she only produces 1)
I think that per 717.1, you would reverse the casting except for Selvala's ability, since it moved cards from the library to hand. Also, the Terastodon would be returned to the top of library.

Is this an “issue” that Selvala would effectively allow you to draw the second card down because you attempted to cast using her instead of activating her ability by itself? Or does it simply play out as I described above?

Edit: Another example: He has a Blightsteel Colossus on top of his library. It is a 2-player game, so it is not possible to get the mana to play it. Can Adam still “attempt” to cast the Colossus so he can draw the second card down with Selvala instead of drawing the Colossus with Selvala?

Edited Paul Lyrenmann (July 24, 2014 01:42:52 PM)

The first step of announcing a spell is moving it from where you're casting it from to the stack. So yes, the first thing you do when you cast the Terastodon is move it from where you're casting it from (the library) to the stack. If you activate Selvala's ability, then find you cannot finish casting the spell, you rewind everything except Selvala's ability (since it cannot be reversed since it moved cards to player's hands). The cards would remain in your hand, and Terastodon would go back to being on top of your library.

Nathan Long
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