North led the Ace of diamonds, cashed the King, then switched to clubs. South won the Queen and switched to trumps, declarer subsequently taking discards on dummy's spades to make 10 tricks: EW +420.
Dummy's 3 of clubs was mixed in with the spades, so the "spades" showing on dummy were KQJT3. Based on this, South believed the second club wouldn't cash (as dummy had played the singleton jack).
The environment was a low-skill club game.
What do you rule?