Here's one I messed up last week:
The auction went (I was very cautious here):
1♦ - 2♣ - pass - pass
Dbl - 3♣ - pass - pass
LHO started with a small ♦ to the J and my Ace. I played a low ♠, LHO played the 4, the K in dummy to RHO's Ace. While she was thinking about what to play next, I realised I missed an extra opportunity here. She switched to ♣Q for my Ace, and I continued ♠ to LHO's Jack. He played a ♣ as fast as he could, and I went -1.
What went wrong? Double dummy, nothing went wrong, I simply can't make it. However, if I played ♠5 in trick 2 (and not the K), I can bring RHO on lead later on, who will have a hard time underleading ♥A (probably holding the K or Q as well). This way I improve my chances if RHO has only 1 ♣ (which you actually have to hope for). I guess I was just hoping too much on a defensive error or luck in ♠...