Allowing declarer to score +680 in 4♥ while the opponents could have cashed an ace-king in two suits, looks to be a blunder. But is it?

Cap Gemini Tournament, 1998, The Hague. NS are Enri Leufkens and Berry Westra, already then veterans in this prestigious tournament. As a pair their best years were '93-'96, but at this tournament they always perform well. They play against two brothers, Jason and Justin Hackett, nowadays a world famous pair, then at the start of their illustrious carrier. The English pair did very well and finished fourth, despite this disaster against Leufkens − Westra.

N/All		♠	9 6
		♥	J 2
		♦	A J 10 9 7 4 2
		♣	6 3
♠	A K 10 4 3			♠	J 8 2
♥	10 7 5			♥	Q 8 4
♦	3			♦	8 6 5
♣	Q J 8 7			♣	A K 9 4
		♠	Q 7 5
		♥	A K 9 6 3
		♦	K Q
		♣	10 5 2

South bid 3♥ for two reasons:
1. Since it shows strength (3♥ is forcing), it might deter the opponents from entering the bidding. After all, from South's point of view EW might very well make a high contract in a black suit.
2. The 3♥ bid tells partner what to lead if the opponents were to buy the contract after all.
Whether South's tactics worked or not, is hard to tell. Whatever the case, EW remained silent and Westra found himself declaring 4♥.

Jason Hackett led the ♠K, his brother playing the ♠2. What was West to do? In view of West's singleton diamond, it was rather likely East had a guard in that suit. If so, the defenders had to prevent declarer from taking ruffs in dummy. So Jason switched to a trump.

Westra played a small trump from dummy and was now able to draw the trumps without losing a trick and cash the diamonds in order to score an improbable +680! Twelve tricks to declarer, while EW had an ace-king in both black suits for the taking.

West's switch was doubly unlucky. Firstly because declarer had ♥AK9: only if South had exactly that (or ♥AQ9) would the switch cost EW a trump trick. Secondly because the diamond suit in NS was solid. If that hadn't been the case, the trump trick probably would have been returned with interest.
A disaster? Yes, without any doubt.
A blunder? No, one can construct several hands in South against which a trump switch is necessary to defeat the contract. An example: