This is a follow up to an earlier question about Selvala: http://apps.magicjudges.org/forum/topic/10471/

To summarize that exchange, if Selvala is activated during the casting of a spell or activation of an ability, that mana ability can't be undone.

There are several circumstances under which a player may start to cast a spell (or activate an ability), activate Selvala, and be unable to reverse the casting processes without first reversing Selvala's ability. The inability to reverse Selvala's ability then creates a scenario where a spell is on the stack that can neither be cast nor reversed. As far as I can tell, the resulting scenario is uncovered by the rules.

Most of these circumstances come up when casting a spell from the library (e.g. with Future Sight or Panglacial Wurm), although there are many possibilities.

There are many simpler examples, but I'm going to include a complex one that shows some extent of the potential rules nightmares:

Nissa has Chains of Mephistopheles and Aven Mindcensor in play. Adrian has Selvala, Explorer Returned in play and Blightsteel Colossus as his only card in hand. Adrian cracks an Evolving Wilds.

While resolving the Evolving Wilds ability, Adrian sees Panglacial Wurm second from the top of his library, underneath Forest. He puts Panglacial Wurm on the stack and activates Selvala. He then reveals Forest (Nissa also reveals Forest, so Adrian adds no mana to his mana pool), discards Blightsteel Colossus, shuffling it into his library, and draws a card face-down. Adrian realizes he has insufficient mana to cast Panglacial Wurm.

This brings up a few questions:
* Panglacial Wurm on the stack, and can't be legally cast. Where should it go? It went to the stack as the 2nd-to-top card of a library that has since been shuffled.
* Which cards (if any) should Adrian now be looking at to finish resolving his Evolving Wilds ability?

1. I'm not sure why this is too relevant. The library is about to get shuffled anyways due to the resolving Wilds ability. Just put the Wurm back in the library (as the second from the top if it matters), then finish resolving the Wild's ability and shuffle.

2. Whatever cards are still in their library. The only card that's not in the library that was in their library is the card that's currently face down in their hand due to the Chains. The Wurm and the Colossus are still in the library at that point.

I'm also going to caution people about asking these types of questions in the future. This is, by definition, a corner case scenario. This isn't a situation that you're ever going to run in to in a game of Magic. This is more of a situation crafted in someone's mind that people won't ever encounter and there's not much to be learned from it. If you have corner case scenarios that you want to discuss in the future, I would advise post in the Rules Theory and Templating forum on the Wizard's forums and not here: http://community.wizards.com/forums/107121

Nathan Long
Wizards.Com Boards NetRep