Take this hand from a few days ago:
At the table, I held up once, then took the Ace of clubs, played the Ace of spades and a heart to the King and played the Queen of spades. East took the King and played another club. So, I lost three clubs and the king of spades. 3NT made 3. A seemingly average board, but had I not missed a clue? The 4 of clubs was the lowest club, and so West is marked with only 4 of them. If I hold up twice and then take the Ace of clubs, East would not have a club to return and perhaps, then, I had a free diamond finesse. Ergo, I should have made four! Poor play!
But looking back, now, If I had done that, without a club to return, East would have led a heart -- the obvious lead, with a void in dummy. And unable to risk the heart finesse (since West held the fourth club), I would have to go up with the Ace. And then, I could not risk the diamond finesse either. So, there are only 9 tricks no matter what. Not scrutinizing the opening lead did not cost.
Or take this hand from a more recent club game:
I tried the club finesse, losing to the king. East returned a high diamond. I played the king. West now took the Ace and played the jack of diamonds to my queen. Now what?
I could see one diamond loser, one slow spade loser and one heart loser. I have already lost two tricks. I gave up on the hand and played a trump. This way, I can get rid of the slow spade loser and get out for down 1.
Immediately, I realized that I had made a mistake. A better line is to now go to dummy and pitch the diamond loser on the club queen. Then, pull trumps. But as you can see, that is no good as the hand lies. West wins the Ace and plays the 4th club, East ruffs the Jack and I don't get a spade discard. So, I am still down 1. This is a better line, though, because it would have worked had West held the second heart or the 3-card club suit. But again, giving up on the hand too quickly did not cost.
You know it is a lucky session when you make your mistakes on boards where they will not cost. And sure enough, we ended up with 60% in both sessions, good enough for second overall both times.