While some of these have appeared in other threads I have started, I will make this hand my first formal nomination:
This auction occurred at three tables, with 5♣ going down one or two. I will add a fourth table here, where (in this sequence) South simply passed the 3♥ bid by North (down five undoubled). What caused this? The treatment of the 3♣ bid: "My long suit - no major fit - 6+♣; 1-♠; 11-16 total points". If South had six or more clubs and 11-16 points, he would have acted over 1♥.
I will add this (similar) treatment of the 4♣ bid ("4-♦; 2-♠; 13- HCP; twice rebiddable ♣; 15+ total points"), which occurred at two tables (down three or four tricks):
Some Souths handled this sequence better than others. The winning call (made at only three tables):
That failed by one or two tricks, even though (double dummy) North-South can make 3♠. One South should have made his preference known a round earlier:
At four tables, South never retreated to spades (down three or four):
Finally, at two tables (mine included - a rare win in the duplicate IMPs), North-South produced a plus score with an "eccentric" bid by South:
One doesn't see both sides playing the same contract at the 3-level very often. This failed by two tricks (+100 and 9 IMPS). While I don't feel particularly proud of the 3♣ bid, it did have the effect of keeping North out of the auction.