This one is from the very beginning, perhaps after 2-3 months of playing. Such boards can't happen again.

K9 J43 AQJT873 Q

JT765 8 96 J9874

AQ3 KQ65 K4 AT52

842 AT972 52 K63

Obviously, we didn't respond to 1 NT with so few points at that time. I lead the 2, north played the jack, and declarer won with the queen. Then he played the Q. North won the trick. I thought "good, now he'll return a heart and I will collect four tricks". But he returned the Q. At first I was angry, but I saw there's something strange going on when west intelligently ducked. You can guess how north played the next six rounds. The declarer discarded a heart in the process, so we won seven diamonds, four hearts and one spade. 1 NT was just made - one trick. My partner blamed me for underleading the ace and allowing the declarer to take that one trick.

AK K863 J754 876   QT AQJT95 A2 K53

This one is from a serious tournament some time around 1995. We reached 4 against silent opponents. The lead was T. I thought: if club ace is on my left then I'll go down. But perhaps the lead was from T9XX? Perhaps I can establish the jack of diamonds and discard a club? So I played low diamond from both hands (RHO played low too). The continuation was small club to the ace, and the diamond ruff. Do I have to mention that none of the remaining players at my seat got the funny idea to let the T hold (I guess they all got the diamond lead - but maybe their RHOs did overcall diamonds), so we were 12 IMPs short on that board?

I didn't play the following board, but I find it very funny.
Two of my friends who don't exactly play bridge (but are more familiar with games like poker) went to a 3rd class bridge club on a session for beginners and were about to face their old adversary from the poker table. So they agreed to cheat against him - strong pass without alert. You can guess the fun starts when they (EW) both pass with 31 HCP total.

N	E	S	W
	PASS	PASS	PASS
2	PASS	3	3 NT
PASS	6 NT	PASS	PASS
PASS

But North was bored and decided that he paid entry fee to play bridge, not to pass, so he opened strong 2, game forcing and ace asking bid. His partner bid 3 showing the ace of diamonds. West finally woke up, East realized what happened, and bid 6 NT.

T865 764 Q543 Q7

AJ3 AQ2 KJ 98654

KQ4 KT3 87 AKJT2

972 J985 AT962 3

Having mirror with 11 tricks, the declarer only had to guess the diamonds (remember that diamond ace was bid by South). But he played low to the jack and went down.

My RHO in the following board was the oldest bridge player in Belgrade. He was over 90 when we played this hand.

Q84 T72 43 AJT76

JT9 AQ63 652 K43

N	E	S	W
PASS	1	PASS	1 NT
PASS	2	PASS	PASS
PASS

Natural, 4 card major. West showed club suit. My partner didn't want to compete with 5 spades in pass-out because he was weak and we were vulnerable.
I lead a trump and declarer played three top trumps (partner following). Next went the 9. Having three cards headed with the king, I routinely played the king (this ensures that declarer takes no more than one trick if he has doubleton club). I can imagine what he must have thought about my skills when my partner delivered the queen in the same trick. He didn't bid two or three clubs although his partner was sure to have clubs, probably because his diamonds were AKQJX, while clubs were just four small.
Result was, needles to mention, bottom for us. Just like a year or two earlier, when Methuselah lead small from KQTXX against my 3 NT. You can guess he found his partner with doubleton ace (and JXX on the table), and you can also guess how frequent this lead was at other tables.

This one is not so sad story. It happens, although not often enough, that luck is on my side too.
I was playing 6 (after pretty aggressive bidding by me):

97 AQ83 AT765 Q4   AT86542 J J3 AK2

Not exactly a comfortable contract. But it is obvious that there's no other normal line of play but heart finesse to discard a diamond, in case trumps break 2-2. The lead was small club. Seeing the board is simple, I made a common mistake - I didn't think. I took the club in my hand, played trumps, and was feeling happy when they broke 2-2. But when the opponents returned the second club, I spotted that I must win it with dummy's queen, and that I have no way to return to my hand to make the heart finesse!
But I didn't surrender. I saw two solutions (except for the doubleton heart king): there is a small chance of RHO playing the K on the second round when I play the ace and show the jack, and, second, perhaps my LHO, who is already supposed to hold the K (or my play makes no difference), has KQ, so that I can squeeze him. After heart king didn't appear on the second round of hearts, I played trumps, and guess what: LHO discarded diamond queen on my last trump! The squeeze worked. I discarded the Q from the dummy and claimed.