You win the opening lead of the ♦5 and play trumps with East winning the second one. East plays a ♦back, you win in dummy and play a ♣ to the Jack and Ace
The trick is quitted and LHO switches to a ♥ which you run losing to the hand on the right. Whilst this trick is going on declarer looks puzzled. East now says "When I played my low club and quitted the trick I think I picked up dummies remaining low club and added it to the cards in front of me."
South now says "But that isn’t fair. I led the Club to get some discards but when West led the heart I suddenly realized that there were no clubs in dummy so I ran the heart in the hope the King was with West otherwise I would have risen with the Ace and taken some discards. I didn't realise dummy was a trick short"
How would you rule? Law 67 only seems to cover you adding one of your own cards to a trick not one of the other sides!
Opinions welcome and it is real life not something made up!