Playing online (anonymous partners, European money site), I held:

6
Q76
Q65
AK8765.

LHO opened 1 and with both sides vulnerable (IMP scoring), my partner bid 3. Everyone passed and I ask you to watch helplessly with me as partner (mis)plays the deal.

Vul:Both Dlr: East	6 Q76 Q65 AK8765
Q54 J54 AJ3 10432		K10 AK109 9874 QJ9
	AJ98732 832 K102 --

See if you can spot the errors. The A was led and everyone played small. LHO switched accurately to a low heart and RHO won the 9. East cashed two more high hearts and then played the Q. Declarer won in dummy and led a low spade. RHO played the king and declarer won the ace. Now, declarer led the 7 (hoping RHO had KQ doubleton). RHO won the 10 and the contract was down 2.

Since standard signalling was used, on the A lead (West led his partner's suit), East played the 4. When South played the 2, it was obvious to West that his partner didn't like diamonds (the 4 was the lowest one outstanding). South should have played the 10? Why, by hiding the low one, declarer might cause West to misread the signal. The 4 might look high (maybe from K9842). As declarer, signal as if you were a defender (high means you want them to continue--because it will make their signal look high).

After South's careless 2, it made it easy for West to switch to hearts. Perhaps he should lead the J, but his low one did the job. East read him for an honor, so inserted the 9 (a key play). Had East won with the K, he couldn't cash all 3 defensive heart tricks. After cashing 3 hearts, East should have played the 4th heart, hoping to promote a trump trick (picture West wish 9xx). When declarer eventually led a trump from dummy, East made a good play of the K. Had he played the 10, South would have played the jack and couldn't go wrong (losing only 1 spade trick). After the K drove out the ace, declarer had a guess. Or did he?

For one, it was unlikely East had KQ doubleton. It looked like he had 10 points in hearts and clubs (from the early play) and 5 more in spades would give him 15 and a likely 1NT opening. More importantly, even if East did have KQ doubleton, declarer would still lose 2 spade tricks. Why? Because East was marked with the 4th heart (remember that he won the 9 at trick two and next played the AK; West couldn't have J10xx--nobody defends that way). So, if East won his now-bare Q, he would just play the 4th heart and West would make the 10 anyway. So, after winning the A, declarer should have continued with the J. At least he would have salvaged down 1.