This hand came up on board 6 of an 8-board match of a country congress swiss teams event.
NS were playing a short club system with a few gadgets including an 8-10 1NT opening 1st/2nd seat favourable and gave thorough pre-alerts at the start of the match. Under ABF System Regulations, an 8-10 1NT range is legal within all system classifications, but a pre-alert is required due to its unexpected nature. When told of the mini-NT, East (a bridge pro) told his partner that against the mini-NT they should double with any opening hand.
East alerted 2♣ and North enquired. East said, "We've agreed to double with any opening hand so this shows both minors with less than an opening hand". West expressed some confusion and/or disagreement with the explanation with a somewhat startled look and a bit of a head shake. After North's pass, East commented "Well, I have to bid according to the agreement here" and passed.
The EW convention card says that 2♣ vs 1NT is any single-suiter, but by a passed hand is both minors.
After the hand, West opined that just because we are using double to show an opening hand doesn't mean we don't play our usual defence to 1NT to which East said, "Yeah, fair enough".
North called the TD suggesting that had he been told 2♣ was a single-suiter, he would've competed with 2♦ and quite probably bought the contract in 3♦ making 9 tricks. East had a bit of a whinge that they've never played against a mini-NT before and didn't have adequate opportunity to prepare.
How do you rule?