I agreed. Sure, it was going to be a bit more mental work, but I thought I'd be able to figure it out. And I was. When partner led the deuce of spades after bidding the suit, I knew he had a 5-card suit. In a few deals, it did help me get the count right. This 3rd-and-5th thing was a pretty easy convention. If any new partner wants to play it, I thought, I'd be happy to agree.
In the last round, though, we ran into this hand. This was the auction (we were East-West):
Partner led the 9 of hearts. This was the whole deal, so you can follow along:
On partner's lead of the 9 of hearts, declarer played a low heart from dummy and I played low (otherwise his 10 becomes a trick). Declarer ducked and partner continued with the 4 that declarer took with his Ace. Next, declarer played two rounds of spades foregoing the finesse ("nine never"). I am forced to discard clubs of course, so I discouraged clubs by discarding the highest club I thought I could afford (the five).
Next, came a heart from dummy. I won with partner discarding a low club (encouraging). I led the three of clubs. Declarer went up with the ace and led a low diamond to the queen. When that finesse won, he ruffed a heart back to his hand and led a club.
This was the situation when declarer led the 7 of clubs:
My usual lead agreements are different. We play fourth highest leads from length, but that is only on opening lead. Here, in the middle of the hand, after I had discarded clubs, etc. my lead would simply be an attitude lead. I would lead a low club from an honor, and a high-club denying one. Since partner has just seen all the honor cards, he would then be able to place me with the Jack.
In other words, playing either convention, the right play is obvious. But once I had led the wrong card, partner had no choice (just as he would not have unblocked had I led the 9 of clubs, denying the Jack if we were playing attitude leads). The moral of the story is to not agree to play unfamiliar conventions -- there are subtle extensions that an expert partner will play you for.