We were playing in the usual pairs game on a July Tuesday at Trumpington. The opponents who came to our table for the last round were not impressed with the general standard. "There are some absurd results on the travellers. On one hand, for example, the whole field was in 6NT+1, with 12 solid tricks and a simple finesse for 13, but one declarer managed only 12!". We nodded gravely at such incompetence, but exchanged a knowing glance with each other  for it was my partner who had made only 12 tricks! Actually, I thought she played it well, though she (& I) missed an interesting point at the end.

South opened 1NT and ended in 6NT, oppo passing throughout. West led the 7.
There are indeed 12 top tricks and there's the easy shot of the spade finesse for 13. But pard saw that if the spade finesse were wrong, and if LHO also held at least 4 hearts, then LHO could be squeezed. To find out more, she won the opening lead in hand and started by cashing the minor suits. When it transpired that LHO had 3 clubs and no diamonds, and therefore 10 cards in the majors, it became very likely that he had longish spades. Then pard continued with the remaining two top hearts, discovering that LHO had started with 6 hearts and therefore 4 spades. The lead is now in dummy. Dummy is down to a low spade and a losing heart; declarer's last two cards are the AQ. LHO's last two cards are known to be a master heart and a spade, and RHO's are known to be both spades. The fact that LHO had started life with 4 spades means that LHO is 4:3 on to hold the SK, other things being equal. Are there any contrary inferences to be drawn from the opponents' failure to bid or the play to date? I don't think so. Ergo, it was correct to play for the drop, not finesse. On this reasoning pard played unsuccessfully for the drop and we got a complete bottom.
That's what I thought at the time. However, my attention was later drawn to Mestel's Law Of Useless SemiYarboroughs, which observes that in many situations, a defender holding a LOUSY hand having signalled a few times, gets bored and simply follows suit or simply discards up the line. This principle can be surprisingly powerful. In the above hand, the fact that RHO has played the 9 and then the J suddenly becomes deeply interesting and significant. The Law suggests that his remaining card in spades is the King. Or, to put it the other way round, if his remaining spade is the 7 (the only other card out), is it likely that he was awake enough to play the 9 and then the Jack from J97?
Of course, the LOUSY Law holds less well in a game where the standard is high, and on this hand an expert RHO might indeed find it routine to play the spades out of order like that. In practice, RHO was an excellent player  as one would expect at Trumpington  so maybe declarer's reasoning was right after all.