I was West, in case anyone hasn't worked that out.
I think I made the mistake of thinking in absolute terms: "If the diamond was cashing, partner would have cashed it, so a spade ruff is the only chance." Instead, I should have approached it like most bridge problems, by estimating probabilities.
I should have asked myself what were the relative probabilities of (1) a 3
♥ opening on a 4711 shape at green and (2) partner's cashing his winners in the wrong order, and playing the wrong spade on the second round. Those were both quite small numbers, but I think I should have concluded that (1) was smaller.
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If this board occurred after the slam where East flew with the ♣A I think you should have gone easy on partner and just cash out
That's a good point, which again I probably wouldn't have considered. As it happens, this was three boards into the match and the opponents had just gone down in a game, so one wouldn't expect partner to be distracted by earlier boards.