106.4a If a player passes priority (see rule 116) while there is mana in his or her mana pool, that
player announces what mana is there. If any mana remains in a player’s mana pool after he or
she spends mana to pay a cost, that player announces what mana is still there.

Akki controls a Scavenging Ooze. Nezumi controls a Kitchen Finks.
Akki taps his Forest, Wooded Bastion and Overgrown Tomb (his only untapped lands) and says "I make black-green-green. Cast Abrupt Decay on your Finks.“
Nezumi, for some reason not realizing that leaves 1 green mana in Akki's pool, lets the Decay resolve and wants to persist the Finks, but Akki says he wants to Ooze it away.
Nezumi calls a judge because he would've liked to do something with Decay still on the stack now that he realized about the green mana left.

So did Akki commit a GRV by not announcing what mana remained in his pool, or is that in this case not necessary because ”make BGG, cast a BG spell" leaves no ambiguity about what remains?

Edited Toby Hazes (April 21, 2015 01:55:52 AM)

The rule doesn't read “announces *unless it's obvious*”, so yes, it is a GRV. Admittedly, a lot of players are unaware, or at best forgetful, regarding this rule. All the same, it is a rule, and Akki commits a GRV by not announcing the G floating.

Now, for the interesting part: do you rewind? Why, or why not?

d:^D

I rewind.

Yes we have passed a decision point that when rewound the game is
going to take a different course the second time. But that is the
decision point that was ruined by the GRV: Nezumi made a decision to
let a spell resolve based on misleading information about the game
state. The entire reason the rule about announcing floating mana
exists is to avoid Nezumi being Gotcha'd here, not letting him make
the decision with that information makes the rule a lame duck.

On Mon, Apr 20, 2015 at 8:55 AM, Scott Marshall
<forum-17681-237a@apps.magicjudges.org> wrote:
> The rule doesn't read “announces *unless it's obvious*”, so yes, it is a
> GRV. Admittedly, a lot of players are unaware, or at best forgetful,
> regarding this rule. All the same, it is a rule, and Akki commits a GRV by
> not announcing the G floating.
>
> Now, for the interesting part: do you rewind? Why, or why not?
>
> d:^D
>
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–
Gareth Pye
Level 2 MTG Judge, Melbourne, Australia
“Dear God, I would like to file a bug report”

I don't think I would rewind, based on Section 1.4 in the IPG (and Riki's recent blog post about backing up):

Originally posted by IPG 1.4:

A good backup will result in a situation where the gained information makes no difference and the line of play remains the same

In this case, the player explicitly said they would take a different line of play after a back-up, so we are basically guaranteed that this won't be the case.

Edited Chuck Pierce (April 21, 2015 03:30:07 AM)

Originally posted by Scott Marshall:

Now, for the interesting part: do you rewind? Why, or why not?

Rewind.

As Gareth said, the reason we have a floating mana rule is to prevent “gotchas” from an invisible object that your opponent can't verify without hearing your opponent provide it. The rule, in this case, protects Nezumi - he may have cast Seedling Charm on the Finks if he was aware that there was G floating, or he may not have, but he wasn't able to make an informed decision on what the game state was.

Neither player has revealed any hidden information, the backup is not complex, from a technical standpoint and we don't have anything moving into and out of hidden zones that could cause things to play out differently. All that's changed here is that Nezumi has accurate understanding of the game state to make a choice. This seems like a case where the game, as-is, is more damaged than the game would be if we performed a rewind and let the game play out again.

I'm 0 for 2 so far today! Thinking on it more, I realize I was focused too much on a too-technical reading of the IPG description for backing up, and not on the general philosophy of deciding whether backing up or not is more damaging to the game state. In this case, as John and Gareth pointed out, the GRV itself is what caused the damage to the game state, so rewinding to that point (especially since it's extremely simple and doesn't involve any hidden information) is much better as we can correct the error so that both players have an accurate understanding of the game.

This is especially true since Nezumi called attention to the error as soon as they were aware of it, so this isn't a situation where both players have been playing with an illegal game state for some time, it's just a single decision (not responding to Abrupt Decay) that was itself a direct result of the error.

Originally posted by Scott Marshall:

The rule doesn't read “announces *unless it's obvious*”, so yes, it is a GRV. Admittedly, a lot of players are unaware, or at best forgetful, regarding this rule. All the same, it is a rule, and Akki commits a GRV by not announcing the G floating.

Akki announced what mana he produced (BGG), then cast a spell that must use exactly BG mana. I would say he did announce he had mana floating, or it is at least obvious.

*tangent alert*
If Nezumi did not catch the “mana generating announcement”, why would he catch a “mana foating announcement” ?
What is stopping Akki from claiming he did say it post decay, Nezumi clearly was not paying attention.

All the rules we enforce could be met with the counter-argument “what if a player break the rule and then lie about it”. That is not a reason to not have those rules, it's a reason to judges to investigate and be able to notice and penalize when someone is bending the truth to get a more favorable ruling.

- Emilien

I'll let them rewind. if I was convinced this is not a bad play.
For Nezumi,
{If} he noticed there is a {G} in his opponent's mana pool.
{Then} he would do something.

{If} not,
{Then} no action.

So the mistake happened at the time Akki didn't tell the floating mana, it is safe enough to rewind.
And this situation can't be abused if Akki announced the floating mana clearly.