In the rules, it says that you can choose which cards from the discard pile that you want to remove from the game to cast your spells... Why does it matter which ones you choose? There's no way to use the discard pile except to cast spells anyway.... Am I missing something?
If you are playing with the Dark Premonitions and Happy Dreams expansion there is a card: "If two green door cards are on the table, take one nightmare card from the discard pile and put it in the limbo pile". So if you are casting spells and can choose cards...get rid of the nightmare cards and it will nullify this card. This has only happened once for me so far, while it isn't strictly mentioned in the rules what happens if there are no nightmare cards in the discard pile when this card is triggered, I assume you just ignore and discard it.