This deal was played (actually misplayed) in an on-line practice match by one of the country's best teams. Vulnerable against not, South held: 532
KQJ4
7
AQ1087. His partner dealt and opened 1, overcalled with 1.
South made a negative double and his partner cue-bid 2. What does that show? It says nothing about spades (neither tells nor asks). It shows a huge hand, presumably forcing to game. Does it guarantee four hearts? Not necessarily. South should bid naturally, 3 for now.

Over this, opener jumps to 4. Maybe he was afraid to bid only 3 thinking it not forcing. In any event, he has shown more than a direct 4 bid, so he must have a hand just short of a strong two-bid. South now made a good bid--he raised to 5. With such good trumps and a decent hand, he is worth a slam try. A bid of 5-of-the-trump suit when the opponents have bid a suit is used to request partner to bid a slam with the opponent's suit controlled. In fact, North not only bids slam, but control-bids 5. This shows? It has to be 1st-round control of spades, and also shows interest (in context) in a grand slam (since it forces to a small slam). South, with great trumps and a source of tricks indeed jumped to 7. Now, plan the play (you will take your partner's hand) on a spade lead:

532 KQJ4 7 AQ1087

A7 A832 AKQ85 K2

After winning the A, declarer tested trumps. If they split 3-2, he would be in great shape. He could draw trump in 3 rounds, then cash diamonds, throwing spade losers. If diamonds are 4-3, a diamond ruff in dummy provides the 13th trick (4 diamonds in all, 5 trumps in all, and 4 top black tricks). Even if diamonds don't split, the clubs can probably be set up.

Everyone followed to the first two trumps, so declarer drew a third round, the 1 overcaller (LHO) showing out. Now what? Declarer tried the top diamonds, but the suit was 6-1! The 1 overcaller had the singleton. Declarer threw dummy's spades on the top diamonds and then turned his attention to clubs. He played the king and then low to the ace. Disaster--RHO showed out! This was the Real Deal:

Vul: North-South Dlr: North	532 KQJ4 7 AQ1087
KQJ109 109 2 J6543		864 765 J109643 9
	A7 A832 AKQ85 K2

What was the error?

I like the auction, and I am happy with the early play, including cashing the top diamonds. But, once diamonds were 6-1, declarer need to take stock. More accurately, he needed to do some counting and calculating. LHO had overcalled only 1 at favorable vulnerability. With 6 spades (along with the known singleton and doubleton), he likely would have preemptively overcalled 2. Since he rates to have only 5 spades, that means he started with 5=2=1=5 shape. Accordingly, after the K drew all low ones, declarer should have finessed the 10 on the second round. This would lose only if RHO had a doubleton J and East had failed to preempt with his 6-card spade suit. The bidding and odds heavily favored the winning play.