I was answering someones question about this yesterday and I wanted to verify a few details.

Donna's triggered power is "Whenever Donna Troy becomes stunned, return Donna Troy to owner's hand."

Now if enough Gotham Knights team attackers to trigger GCPD Headquarters attempt to stun Donna, which triggered effect goes off first? "Whenever three or more Gotham Knights team attackers you control stun a defender, KO that defender."

My reasonings were from the CGR that states: "504.2 The player that plays the effect is the controller of that effect. If an effect is triggered, the player who controlled the object that triggered an effect, at the time it triggered, is the controller of that effect."

So my way of thinking was that the controller of GCPD Headquarters controlled both triggered effects and could decide the order.

Thinking it and rethinking it is starting to make me question that. Paticularly, the "becomes" in Donna's text is bothering me. Does Donna even stun before she returns to the hand?

When multiple triggered effects trigger at the same time, the primary player places their effects on the chain first, then followed by each other player.

If the GCPDs are attacking Donna, then the player controlling the GCPDs is the primary player.

GCPD HQs effect is placed on the chain first, followed by Donna's effect on the chain. The chain resolves last in, first out. So in this instance, Donna's effect will resolve first, and she will be returned to hand. Then the GCPD HQ effect will resolve without doing anything.

Hmmm,

Isn't the Gotham Knights player in this case the controller of these two effects as per Rule 504.2. Isn't he is the one responsible for triggering both effects?

Also, one of the things that was bothering me was that Donna's text differs from Toad's and Kevlar Body Armor's in that she doesn't use the phrase "if' she "would become", but the phrase "when" she "becomes".

I realize this is a subtle difference but I've learned not to take the subtle differences in this game for granted. They've always been more precise with the card text then in some other games.

If Donna is like Toad, then wouldn't she not stun, and would have to be returned to the hand before GCPD HQ could resolve? I don't view her text this way myself, but clearly I'm missing something.

No, the GK player does not control both effects.Together with the above, 504.2 States that GCPD HQs controller is the controller of that effect. And Donna's controller is the controller of her effect.

Donna has a triggered power, whenever she becomes stunned, she places a triggered effect on the chain. Her controller controls that triggered effect she placed on the chain.

GCPD HQ also has a triggered power that triggered when Donna becomes stunned, and that too places a triggered effect on the chain. Because we now have 2 events triggering simultaneously, 703.3 quoted above will determine the order of which the 2 effects are placed on the chain.

Toad and Kevlar Body Armor mainly differ from Donna because of the word 'instead' in their effect. They have replacment modifiers.
If say 3 GCPD officers attack Toad, and Toad would become stunned, Toad's controller may return Toad to hand instead. If he does, then GCPD HQs effect does not tigger, as Toad never did get stunned, that stun was replaced (and also why he does not take stun endurance loss).

And to answer your other concern, Donna Troy is stunned before returning to hand, meaning the controller of Donna Troy will take stun endurance loss.

Well, that puts another chink in my reasoning.

I'm still missing something here. 504.2 very clearly says: If an effect is triggered, the player who controlled the object that triggered an effect, at the time it triggered, is the controller of that effect."

Since its the Gotham Knights attackers that are responsible for stunning Donna, how is it their owner is not considered the controller of that effect?

Donna Troy triggered her effect by becoming stunned. The character or characters that stun her aren't considered to trigger her effect.

Your attackers did not trigger the effect.

Donna Troy's card text triggered the effect. Therefore Donna Troys controllor controls the effect.