It seems to me that if your opponent had seen this coming, he would have bought that scoring card ahead of you, leaving you with a bunch of useless cubes and octogons at the end of the game maybe? After all, if you were able to aquire that many pairs, I am guessing that you sacrificed some other scoring aspect of the game no?
Exactly. Unless you're playing with new players and can plan this from the start without anyone really knowing what you're doing then this can be easily blocked. Same for many of the potentially big point scoring cards.
Not an end-game scoring card but I tried the same aiming for the card that allows you to convert gold to VP on a 3:1, but again I got spotted in the last 2 rounds and then one guy purposely made all the high value dresses. Otherwise I would have totally spanked everyone in points - I had well over 100 coins if I remember rightly.
This is my one main gripe about the game. The end game scoring cards would have been better off coming out in round 1 to act as a game goal card for a player. Coming out in round 3 or 4 (or whatever) it's a massive gamble.