Observed at a GP:
AP: I cast Abzan Charm on your Dragonlord Kolaghan.
NAP: Okay, but you have another Charm in your graveyard, right? Kolaghan triggers.
AP: Oh geeze, that's bad. Umm, let me think.
Me: Hey guys, let me pause you here. Kolaghan actually only triggers off creatures or planeswalkers, not instants.

Why this was interesting:

At first glance, this looks like a simple GRV–Kolaghan should not have triggered, but did.

However, Kolaghan's trigger is one that creates a change in the visible game state. That means even if it's placed on the stack, the trigger can still be missed if the player fails to “take appropriate physical action” when the trigger resolves. At the time, I decided that it was not appropriate to issue a GRV for that reason–how could I claim that a game rule was broken for a trigger that might not even resolve?

Why I felt the need to issue an infraction:

The trigger would have killed the AP–unless he cast a spell in response to the nonexistent trigger to save himself (I think he was running Foul-Tongue Invocation, but I don't remember for sure). Either the AP casting a spell or the AP scooping his cards would have damaged the game state, possibly irreparably. However, the General Philosophy section of the IPG makes it clear that “Judges don't stop play errors from occurring,” so I felt that if I was stepping in, that was de facto evidence that a play error had occurred. Is it possible for a player to make a “play error” that isn't an infraction? Even if it were, on the off chance that this player was trying to trick his opponent, I thought I needed to have this tracked in the system.

What I ended up doing

I actually called this a CPV. My justification at the time was that because the trigger hadn't been resolved, no game rule was broken. However, the active player was incorrectly describing that stack, which is free information, since he claimed there was a trigger when Kolaghan had not actually triggered. Since I intervened to prevent the active player from “acting on incorrect information provided to him or her by his or her opponent,” which would be grounds for a CPV backup, I felt like this was a good compromise.

But in retrospect, this seems kind of forced. What's the difference between “putting a bogus trigger on the stack” and “describing a bogus trigger that's not really on the stack”? I feel like this logic could be used to twist any number of GRVs into CPVs by declaring that a rule wasn't really broken, just mis-characterized as having been broken. As a result, I'm not confident this ruling was correct.

To sum up
I'm pretty sure that I overthought this ruling. A trigger was illegally played, which is a Game Play Error not covered by other specific infractions. But it still feels weird to me to issue a GRV for a game rule that might not actually end up broken. What are your thoughts?

I just give a GRV here to NAP (and FtMGS to AP for allowing it) for triggering Kolaghan when it should not.

The visible gamestate change is a criteria for determining whether a trigger is missed, but there should not even be a trigger here. The fact that NAP is adding a non-existent trigger to the stack here seems reason enough for a penalty.

Maybe I am oversimplifying the situation though, I look forward to what other people have to say.

Both players have agreed that the Kolaghan's trigger is on the stack - it shouldn't be (even if it doesn't resolve) - that's enough for me to apply a GRV/FtMGS.

As Federico Donner once told me: “It's never CPV.”

Well, except when it's obviously CPV.

This looks like a straightforward GRV for NAP and FtMGS for AP. You stepped in at an appropriate time as an error had already been made in that both players were in agreement the trigger was on the stack. The fact that the trigger may not even resolve is irrelevant, as the error committed was putting it on the stack in the first place.

Edited Chris Wendelboe (Dec. 4, 2015 08:44:30 AM)

As Frederico Donner once told me: “It's never CPV.”

I almost wrote an email just saying “It’s never CPV” but felt like trolling wasn’t the best course of action. Thanks for doing it for me :P

Chis Lansdell made a great presentation on CPV for a conference, I tried finding the slides to link here but I couldn’t so I summon Chris to do it himself. 

In this case, I wouldn't assign any infraction to AP. FtMGS doesn't mention anything about accepting the opponent's incorrect assentation of the game state, and we intervened before AP had the opportunity to process the statement.