During a club multi-teams event, I was sitting west when this hand arose. Partner's 2NT was a surprising bid and I did not immediately recall the meaning. After a little thought I remembered that we had agreed to play that an immediate 2NT would show a balanced 17-19, but after an initial pass 2NT would show the minors. This partnership agreement, that a delayed 2NT would show the minors, has been in place for several years but had never occurred at the table. My alert was made after the 3♠ bid and South was given the option to change her call, but she chose to still bid 3♠ (and chose not to ask the meaning of 2NT).
What should I bid? There was good reason to suspect that partner has forgotten this part of the system: (1) it has never occurred at the table and (2) I have ten cards in the minors and it seems unlikely that partner also holds 10 cards in the minors. I chose to pass, based on the judgement that partner had probably forgotten the system and if her bid were in accordance with the system, the opponents would hold a very big fit in the majors.
North now asked the meaning of 2NT and my explanation: "I believe we agreed that ..." was interrupted and North thought it wisest to read our system card. The system card confirmed that we had agreed to play a delayed 2NT to show the minors. North elected to pass.
The table result was 3♠-1 a 3 IMP gain for N/S as East played 2NT+1 at the other table.
But I was interested to get opinions whether any infraction occurred?