A subtle defensive problem by Barry Rigal

Love All. Dealer North.

                           S --
                           H AK974
                           D 9632
                           C T763
                S QT84                S 632
                H JT3                 H Q865
                D A7                  D KQJ54
                C 9842                C 5
                           S AKJ975
                           H 2
                           D T8
                           C AKQJ

Versace (Italy) plays 4S from the South seat after Gromoeller (Germany) had bid diamonds in the East and go quietly down one.

In the Closed Room, this was the auction:

             West      North     East     South
           Lanzarotti   Reps    Buratti  Rohowsky
                        Pass     Pass       2C
             Pass        2H(1)   Pass       2S
             Pass        3H      Pass       3S
             Pass       3NT    All Pass
            (1) 5-7 HCP

3NT by North was the final contract - which of course, is trivially down one on a low diamond lead - but Buratti could not see through the back of the cards and led the DQ, showing either D KQJ or D QJ without the ten.
Lanzarotti ducked this and overtook the DK with the Ace. (Had he overtaken the first diamond Reps might have false carded on the second round of diamonds, with the D6 or even the D9, to leave East with a very difficult problem.
Buratti now found the crucial switch to the HJ, and Reps won the Ace to play the D6.

This was the position:

                            S --
                            H K974
                            D 96
                            C T763
                 S QT84               S 632
                 H T3                 H Q86
                 D --                 D J54
                 C 984 2              C 5
                            S AKJ975
                            H --
                            D --
                            C AKQJ

Buratti won the DJ, and the key to the defense was that Lanzarotti pitched a heart. This allowed Buratti to exit with a spade, and declarer could do nothing.
If he wins the Ace, cashes four rounds of clubs and exits with the SJ, West can win and play a heart and declarer has
to concede a heart at the end.
If Reps only takes three clubs, then West can win any spade and exit with a club to dummy to collect another spade trick at the end.
But if Lanzarotti had pitched an apparently harmless club on the DJ, Reps can succeed if he reads the position. Say Buratti plays a spade now. Reps wins the Ace and plays three clubs.  In this ending, declarer has five tricks.

                            S --
                            H K97
                            D 9
                            C T
                  S QT8              S 63
                  H J3               H Q8
                  D --               D 5
                  C --               C --
                            S KJ97
                            H --
                            D --
                            C J

Reps exits with the SJ, and West must win his Queen. He can either concede the rest to dummy or lead a heart and allow
declarer to take his two red suit winners, to pitch the spade losers from dummy.