The following came up in a game the other night.
Mage of Loec reads:
Action: Cancel an Action just triggered. Then, this unit becomes a facedown development unless you discard a quest from your hand.
What happens if each player has a Mage of Loec? Say Player A triggers an Action, Player B then triggers Mage of Loec's Action to cancel that Action. Player A responds by triggering her own Mage of Loec to cancel Player B's Mage of Loec. Player B then responds by triggering her Mage of Loec to cancel Player A's Mage of Loec, ad infinitum. Does this go back and forth forever?
The second sentence, "Then, this unit becomes a facedown development ..." appears not to be a cost of the effect, but part of the effect itself. If the card read "Action: Turn this unit into a facedown development or discard a quest card from your hand to cancel an Action just triggered" it would be a cost and so would have to be paid at the outset. Meaning it could not go on forever, assuming the players did not have a way to discard an infinite number of quests from their hands or turn Mage of Loec back into a unit from being a development. However, that is not how the card is worded.