Things to keep you awake at night. From "Making of the Atomic Bomb" by Rhodes.
Fat Man, unit F31 was the second unit assembled. unit F33 with a non-nuclear core was due to be dropped on 5th Aug but was finally dropped on 8th Aug as the test drop crew were delivering Little Boy to Tinian.
Fat Man was due to be dropped on 11th Aug, but bad weather was forecast over the target for 5 days after the 9th so Tibbet persuaded them to bring it forward to the 9th. In order to do a lot of pre-planned checkout procedures were taken out of the assembly process.
On the night of 7th Aug an ensign, Bernard O'Keefe, had the task of doing the final check of the bomb before it was encased in armour and became unreadable. Part of the checkout required connecting the firing unit on the front of the bomb to the with the radar units in the tail with a cable which wound around the implosion sphere. When he went to do so he found two male connectors at one end and two female at the other - the cable had been installed the wrong way round, and reversing it would mean dismantling the bomb taking days.
So, without telling anyone, he called a technician, ran 2 extension leads from adjacent lab, and unsoldered, reversed and resoldered the plugs on the lead; ran some continuity checks - then tidied everything up, finished connecting everything up and finished sealing the bomb. He didn't tell anyone till after the war......