a)  Here South has no great need to be on lead and so should let the Q♣ go.  The complete deal (from Round 3 of the 1989 NEC Bermuda Bowl) is as shown.  If South ducks the Q♣, North wins and should switch to the J♠ to the Queen, King (South cannot gain by ducking) and Ace.  When South comes up with the A♣, the 10♠ is cashed and a third spade saddles declarer with one spade and three club losers.  Curiously, at three tables (New Zealand, Colombia and Canada), South took the Q♣ with the Ace.  Now declarer won the K♦ and led a club to the Jack (one North ducked this!).  Later declarer ruffed two club losers, ruffed a heart and led the Q♠, holding the losers to two clubs and one spade.

b)  Four declarers went off by playing the A♠ first, but the Egyptian declarer succeeded against Australia.  At trick two he led the Q♠ and the Australian South ducked.  Had he covered, he would have been able to draw dummy's trumps and defeat the contract.  After the Q♠ won, declarer cashed three diamonds, ending in dummy and led a club to the Queen and Ace.  South led a spade, taken by the Ace, and a second club was led from dummy.  The defence could not prevent declarer making one club trick and ruffing a club loser.  Although it was unlikely that North would produce the J♠, South could not be in a better position by declining to cover the Q♠ if East did have Q-J-x-x-x-x.