It is very easy to get stuck on one idea at the bridge table and not consider alternatives, or not change your plan when new information arises. I fell from grace on one hand, and my partner did on another hand at Adventures in Bridge on August 29, 2018.
East: ♠KJ5 ♥KQ98 ♦Q10542 ♣Q
West: ♠Q106 ♥AJ6 ♦AK9 ♣K1062
I opened 1NT as West. Partner made a Stayman inquiry and we ended up in 3NT. The lead was the ♣7. I was guilty of sloppy thinking. RHO took the ♣A and returned a low club. The Rule of 11 made it possible that the ♣7 was a fourth best lead, presumably from ♣J987(x), so I foolishly thought the club finesse would fail. Instead of inserting the ♣10—which is an almost no-lose play—I hopped with the ♣K—worried that in case diamonds broke badly, I would need LHO to have the ♠A and needed to keep my ♣10x in clubs as a second stopper against the ♣J on my left. I was duly punished as LHO had led second highest from four bad and RHO had finessed himself on the club return. If I had just played the ♣10, I would have made 11 tricks. Instead, I only made 10 tricks and an apologized to my partner.
South (Dummy): ♠A65 ♥J92 ♦A9753 ♣53
West: ♠87 ♥AQ85 ♦42 ♣AQJ76
This was a case of partner not adjusting her thinking to a slightly different card combination. I led the ♥3 against 1NT by North (12-14 range). Partner remembered the principle of “When they only have one stopper in NT, make them use it up as soon as possible while you both still have communication with each other.” So, she played the ♥Q. This would have been an excellent choice if Dummy had three small hearts and she would have survived if I had led from the ♥K, but alas, I had led from ♥10xxx. Declarer happily took the ♥K and now had a second heart stopper with ♥J9 in Dummy. Declarer proceeded to reel off five spade tricks, and I threw away two hearts—a clear signal that I am abandoning that suit. Partner was still thinking that hearts was our suit, so she threw away clubs instead of hearts and Declarer got two extra tricks in the end.
So, to remind us all: (1) Double check your first plan or early assumptions. Ask yourself if you could be wrong, if there is another possible interpretation of the data you have so far. Try to have at least a Plan B as well as a Plan A in case things go wrong. (2) Stay very flexible—don't get stuck in one defense, or one line of Declarer play. Be able to adapt if new information shows a better way to go.