Match 1, Board 13, all vulnerable, North deals.
♠ Q432
♥ 72
♦ A42
♣ Q874
♥ 72
♦ A42
♣ Q874
♠ K85
♥ Q1043
♦ 1098
♣ 1092
♥ Q1043
♦ 1098
♣ 1092
♠ AJ76
♥ AK9865
♦ J
♣ 63
♥ AK9865
♦ J
♣ 63
♠ 109
♥ J
♦ KQ7653
♣ AKJ5
♥ J
♦ KQ7653
♣ AKJ5
The auction proceeds:
North East South West Pass 1♥ 2♦ 2♥ 3♦ 3♥ ?What would you do as South?
You have a pretty good hand for diamonds and as the cards lie can make 4♦. But also as the cards lie East can make 4♥.
However South is not looking at all the cards. You should reason as follows:
1. Because 3♦ is just one step below 3♥, the opposing auction is ambiguous. They did not have a way to distinguish between competing for the partscore and inviting game -- unless they play maximal doubles, which they don't.
2. While 4♦ seems likely to make, you will not be happy to then hear 4♥. It's wiser to pass and hope the interference was successful. Another option is to bid 5♦ immediately like you think it will make and hope for no double.
Sometimes it's best to let sleeping dogs lie. Competing to 4 of a minor is rarely a good idea.