Source: ACBL
Sitting East, you pick up a hand with a bit of potential: 
A 10 9 8 
7 6 4 
A 6 4 
A 5 3
North, however, deals and opens on your right. (Both sides are vulnerable.) You pass and South bids 2NT. North raises to 3NT ending the auction.
Your partner leads the 2, a lead that you rather like. When the dummy comes down you should take a moment or two to review the bidding and see If you can draw any conclusions.
What do you learn by doing this? Form an opinion and continue.
The first thing that comes to mind is that North might have opened 1NT. You catch a look at their convention card and discover they are playing weak notrumps. You then note that 2NT showed 11 or 12 points.
The next thing you should do is estimate their combined assets and work out what your partner can contribute. Dummy has 15 high points and declarer has at least 11. That adds up to 26 points. You have 12, which tell you that your partner has a point or two. No more, no less.
Declarer calls for the 4 from dummy. Now what?
In your style of leads, your partner’s 2 lead promises an honor. Your agreement is that with a bad suit, the lead should be a higher spot card, suggesting nothing good in the suit. The normal play is the 8, forcing out an honor from declarer. Then, when your partner gets in, another spade lead will allow your side to cash three more spade tricks.
That’s all good in theory, but here is what happens if you play the 8: Declarer will win the J, and he will attack clubs by leading to dummy’s jack. You win. Now, can you get partner in to lead a spade through dummy?
Given that you know your partner has a maximum of 2 points, likely the Q, there is no way you are getting him in for a spade continuation.
Here is the complete deal:
Your playing the 8 was well intended, but it didn’t work. Declarer will make at least nine tricks and probably will make 10. He might even make 11 if you fail to take your A at some point.
Could you have beaten 3NT? In fact, there is a successful defense.
Win the A at trick one and continue with the 10. If your partner has the queen (possible), you can set up three
spade winners and you will get in to use them. This defense goes against the grain, but once you see that playing the 8 at trick one won’t work, you need to look further to see if there is a lie of the cards that does work.
Interestingly, it might seem that partner’s spade lead was necessary to set 3NT. It wasn’t. In fact, any card West leads will work. South has to defend correctly, but in all cases, there is a winning defense. Say West leads a heart, the most passive lead he has.
Declarer wins and eventually leads a minor suit East takes the first round of whichever minor declarer is leading and leads the 10. This play traps South’s jack and it sets up three spade tricks for the defense without giving away a ninth trick in the meantime.