A "lock" is a sure thing, a colloquialism among bridge experts. If you play too fast and do not take those extra chances, you may be missing a lock, as I did here.
After a passed hand Drury bid by partner, showing a good 3 or 4 card spade raise, I bid the game. The Queen of clubs was led, won with the Ace in dummy, then followed quickly, club ruff, heart ruff, club ruff, heart ruff (Queen appearing, no idea why)
I now cashed the SK both following, and tried a diamond up to the King. It lost to Lefty. Lefty proceeded to cash DQ, and exited with his last club (a diamond or a spade or a heart hands me the contract). I ruffed, and took stock. When I cashed the Spade Ace the trumps failed to behave and the position was this.
If I had been playing more slowly, I would have realized there is now no need to bank on H"AJ" bringing down a now stiff HK (despite the falsecard). I had a lock! I should play the trump exit (unusual in a cross ruff type of hand), since a) there are no club masters outstanding b) DJ and the club in dummy will either spring to life or I will get a lead into my H:AJ.
Instead, I sadly played the other major suit ace and went one down.
EDIT: If west keeps his HQ, I should ruff out his card of exit, clubs and then play for the endplay which works because Lefty holds all the diamond honors. If I fail to ruff out his card of exit and play a diamond first, he takes his two diamond tricks and exits with HEART QUEEN, I may win but the endplay or stepping stone does not work because he has a card of exit, a club and I must additionally lose a heart to RIGHTY.