I wondered about writing a pastiche, a "Muck these hands up with me" sort of blog.
Playing in a robot express duplicate I finally found a hand that annoyed me so much I offer it up here.
Sitting west in the following hand
The A♦ was led from north and I duly lost 1 ♠ and 2 ♦ tricks. There didn't seem much that I could have done differently so I settled back to wait for an average-ish score. What I actually got was 20%. There were 8/10 pairs playing in 4♥ and 5 of these had made an overtrick. From a defensive point of view I have found it very helpful to see how this happened.
2 pairs played AKQ♦ and promoted the J♦ in dummy
1 defensive pair doubled a contract of 3♥ then did the above
the next north played 2 rounds of diamonds and then 3♠ so south went up with the Q♠ to promote the J♠
2 norths played 2 rounds of ♦ and then declarer cross ruffed and cashed his winners coming down to an ending with just AK4♠ in his hand but south discarded down to Q 10♠ and K♣ so dummy's J♠ came good.
Sad to say that I became so obsessed with not promoting cards in dummy that later on I failed to cash a defensive winner twice in order not to promote a card in dummy and let a contract make that should have gone down 2 .. a hand so shameful I cannot bring myself to post it.
The messages though are
1) that one needs to beware of gratuitously promoting / unguarding potential winners in dummy.
2) it is astonishing how easy it is to make such mistakes. When declaring just running the winners out allows the defence the chance to mess things up. Here 2 souths kept a redundant K♣ to defend against a squeeze that didn't exist.