Had a complicated (for me at least) director call this evening, and in hindsight I'm not sure my ruling was best.
Neither 2♦ nor 2♥ were alerted. Before East's second bid, he asks North what 2♦ was. He was told "I don't know, but I'm treating it as natural".
East then looks at NS convention card, and sees that 2♦ is systemically showing the majors. He then bids 3♦.
I was called mid-way through the play, and told there has been misinformation. I ascertained the facts above, play continued and declarer goes 1-off, and a ruling is asked for. I checked what South thought his 2♦ showed, and was told "natural constructive."
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I ruled that although there has been MI from North to East, East protected himself by looking at the convention card (and so his random 3♦ bid stays). The fact that NS both have forgotten their system is unfortunate but rub of the green because South has no UI from North (only that North doesn't know what the bid is, and is treating it as natural, which suggests bidding on rather than passing).
In hindsight, I'm not so sure if I should allow south to pass 3♦. South's initial bid of 2D is weird, and when partner bids a presumably-natural 2H, how can you not raise with undisclosed 5-card support. Does south have UI that because north doesn't know the system, he might think it's majors, and so passing 2H is an unauthorised tactical decision that should be adjusted? Is it just ironic that the "UI" that partner doesn't know the system has woken South up to that his bid shows Majors and he has made an "unauthorised panic"? But the UI that partner is "treating it as natural" suggest natural hearts, and so bidding on might suggested by this UI!
My judgement of the situation might have been swayed by East inferring that "rub of the green" rulings when *both* players forget their system allows cheating to prosper. I interpreted the tone of this comment to be an accusation of improper conduct (basically calling NS cheats without saying so explicitly), so I gave East a disciplinary penalty.