The exact setting was that we were leading by a small margin over two teams in the last round of a sectional swiss. Over 4♠, I thought my hand was worth another try, but I was not thinking very clearly. I tried RKCB 4NT rather than the far superior 5♥. My partner bid 5♦ showing one key card. This was doubled. I signed off in 5♠, assuinng that partner had the heart ace.
These were our two hands:
Diamond lead to the Ace and Club Queen shift at trick two. Down one.
If I had chosen 5♥ as my slam try, partner would have signed off in 5♠ and we might not have gotten the diamond lead. For that matter, if we were playing 1430 RKCB, partner's response would have been 5♣, avoiding the lead directing double and again possibly avoiding the diamond lead. And this is beside the fact that it is somewhat unlucky to find the ♦A in RHO's hand and the ♣A in LHO's hand.
A further point - the opening lead (from 3 small diamonds) was the ♦3. RHO, on winning the ♦A, saw my ♦2 play, so he considered the possibility that his partner might have the ♦K. However, he worked out that he needed 3 tricks to beat 5♠ so he eventually shifted to the ♣Q. But he did mention that if I were in 6♠ he would have a much more difficult problem.
LHO thought that his partner might need count on the diamonds, hence his low lead from 3 small (playing 3rd and lowest leads).

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