I got this wrong, but perhaps it is of interest. I'll make a comment or two about discarding agreements, and I'll be interested in any thoughts.
Matchpoints, casual pick-up, std cding.
Partner led the club 3 to the 5,J,K
Club 6 on my left, 2,A,?
I suppose that clubs began as 5-5. Lho presumably holds at most three spades. I will, for now, play declarer for 3=2=3=5. I figure to let him break his own spades. I play the spade 4, standard.
Comment: I have never agreed with the claim that udca allows you to hold on to your high cards. Rightly or wrongly (and it was right) I want to discourage spades and I am happy to be able to play the 4 to do so.
Heart 5 from dummy, 2,A,4. Some evidence my shape guess is right.
Club T, 4, D2,H3
Club 9, Q, S3, ?
And here is where I made my error, playing the D7.
Club 8 from partner! H7,D3, C7! (Declarer presumably lost track of club spots but in fact he had no good options.)
My trusting partner now played the king of diamonds, holding, and then a small diamond into declarer's AJ.
My error. If I play my diamonds low up, there is at least a shot he will work out to play a heart and we will beat 1NT for a very good score.
After the hand partner commented about how he really prefers o/e discards. Yes? So if my 4 were a 5 I have to pitch the spade T at trick 2? Rather not.
At any rate, the real issue here, as it is so often, is selecting the correct message to send. The sending part is often easy enough.
Thank you for your patience with this.