I hate to harp on about counting the hand (well actually I don't, because it's the single most important thing in Bridge, literally) but here is a really good example where Opps completely (1) failed to grasp the importance of good signalling, (2) failed to grasp what was going on on this hand, and (3) was guilty of now counting his tricks.
You are East (NS Vulnerable) and Opps (Brian Meadows and me) are playing the Complex version of OCP and have bid to 3
on the following sequence:
South West North East1
No 1
(1) 2
2
(2) 3
3
All Pass
(1) 8-10 HCP any shape
(2) 5+ Diamonds
Partner leads the 7
and you can see
Dummy 
542
AJ764
K87
J10
You AK10
KQ10
94
AQ842
You take the Ace of Clubs and return the Queen, which is won by Declarer's King (Partner plays the 9). Declarer now draws 3 rounds of trumps ending in hand. For some reason you discard the
10 on the 3rd round (maybe requesting a Spade, but perhaps upside down suit preference for a Club). Declarer ducks a Heart to your 10 (Partner plays the 8 ), you play the
8 and Declarer ruffs and ducks another Heart to your King (Partner plays the 9). How do you play?
It might seem attractive to force out Declarer's last trump with another Club, on the basis that Partner's 8 and then 9 of Hearts suggests they have 3 Hearts (which means Declarer hasn't got any more), but (1) this flies completely against any logic (why would Declarer abandon the A
in Dummy) and (2) You completely missing the point about this hand if you do that. Consider:
Declarer definitely started with 5 Diamonds and 2 Clubs and has already played 2 Hearts. They can only have a
maximum of 3 Hearts, which means they
must have 3 Spades (4 if they only have 2 Hearts). You have already won 3 tricks, so the AK
must be cashing to take them 1 off. The big danger if you plod on with Clubs is that if Declarer has 3 Hearts, he can now ruff and run 3 Heart tricks to
make his contract.
East fumbled the ball here by continuing with Clubs, allowing Brian to bring home his "impossible" 3
contract. in fact, if East had discarded a small
Club on the 3rd Diamond, EW can actually cash
three Spade tricks to take this contract -2. On the first Heart, it's important for Partner to play the
9 rather than the 9. You can immediately tell that Partner has an
even number of Hearts (a doubleton for certain) and you can
immediately abandon the idea of forcing Declarer in Clubs in favour of cashing your Spade tricks before they disappear.
