Ann played Dromoka's Command. She tapped mana, shows a card, then said:
“I put a counter on my creature and made by sacrifice artifact or enchantment”.
Opponent answer with: “you know, that Command's option is sacrifice an enchantment only”.

So Ann wants to take back this play. She argue, that she chose a non-existing mode of the card, so she is unable to finish casting spell, so the action should be reversed.
Do you agree with this argumentation? If not, and if you keep Ann to playing Command, do you let her change the “sacrifice” mod to other?

I think there's an issue with this situation.

Lets put the rules involved here:

Originally posted by 601.2b:

If the spell is modal, the player announces the mode choice (see rule 700.2).

• 700.2b
  The controller of a modal triggered ability chooses the mode(s) as part of putting that ability on the stack. If one of the modes would be illegal (due to an inability to choose legal targets, for example), that mode can’t be chosen. If no mode can be chosen, the ability is removed from the stack.

601.2e
The game checks to see if the proposed spell can legally be cast. If the proposed spell is illegal, the game returns to the moment before the casting of that spell was proposed (see rule 717, “Handling Illegal Actions”).

• 717.1
  If a player takes an illegal action or starts to take an action but can’t legally complete it, the entire action is reversed and any payments already made are canceled. No abilities trigger and no effects apply as a result of an undone action. If the action was casting a spell, the spell returns to the zone it came from. The player may also reverse any legal mana abilities activated while making the illegal play, unless mana from them or from any triggered mana abilities they triggered was spent on another mana ability that wasn’t reversed. Players may not reverse actions that moved cards to a library, moved cards from a library to any zone other than the stack, caused a library to be shuffled, or caused cards from a library to be revealed.

  717.2
  When reversing illegal spells and abilities, the player who had priority retains it and may take another action or pass. The player may redo the reversed action in a legal way or take any other action allowed by the rules.

Considering all of this, I'll allow Ann to go back before casting Dromoka's Command with no penalty a penalty of GPE-GRV to her, explaining about reading a card before make an ilegal choice on it.

Greets!

Edited Cristóbal Vigar Guerrero (May 9, 2016 08:51:42 PM)

Originally posted by Cristóbal Vigar Guerrero:

Considering all of this, I'll allow Ann to go back before casting Dromoka's Command with no penalty to her, explaining about reading a card before make an ilegal choice on it.

Why would you not issue a penalty? Did she not commit an infraction when she chose a non-existing mode?

I think, but I'm not sure, that 717.1 and 717.2 is here to deal with some rare case where the game allowed to take an action that become illegal, like trying to pay the mana for a spell with Selvala, Explorer Returned's mana ability and finishing with not enough mana to pay.

For the case of casting a command with an inexisting mode, assuming thé player is not cheating, I would perform a simple backup before the spell is cast and give a GPE-GRV warning to the player.

Edited Pascal Gemis (May 9, 2016 08:50:03 PM)

Originally posted by Sebastian Reinfeldt:

Why would you not issue a penalty? Did she not commit an infraction when she chose a non-existing mode?

Let's consider an example scenario before answering, because I think you're right.

What happen if Arnold cast a Cryptic Command and choose, Draw a card and Destroy all permanents, and Nathan allow him to resolve his spell.
In this scenario, after solving the weird Command, Arnold has commited GRV and Nathan FTMGS if they discover the problem after the spell resolution, but not before the problem get un-fixable. (Obviously they just commited a mistake)

So, yes, I have to apologise here and change the ruling to, issue a penalty of GPE - GRV only to Ann.

Thanks in caught my error Sebastian.

Greets!