The last unit to flee across the Naktong river had a problem, T. R. Fehrenbach explains (in This Kind of War):
As the rear guard came across the bridge to the east side, throngs of Koreans followed them, filling the bridge with jostling bodies. General Hobart Gay, who had ordered the bridge to be sent up only at his express command, instructed them to go back to the far side, and clear the bridge.
This they did, as dusk approached. Then, with the refugees pushed back onto the west shore, the rear guard turned and pelted across to the friendly bank — but the second they turned, the Koreans dashed madly for the bridge and soon filled it, even before the cavalrymen were across.
Three times, at Gay’s order, they repeated the maneuver, without success. Short of shooting them there was no way to keep the Koreans from using the bridge. Even telling them it would be blown did no good.
Now it was growing dark, and the Inmun Gun was closing. As the rear guard recrossed to the east side for the third time, with the mass of Koreans close behind them, Hobart Gay, his face pale, said, “Blow it.” He had no other choice.
Several hundred Koreans went into the river with the bridge.
Some armchair quarterbacking here: what if he had allowed the peasants to cross without their luggage?
My thinking: you can do concealed carry with small arms, but not with serious weaponry. Any disguised enemy combatants would come over either unarmed or inadequately armed.
Oh, and… frisk them afterwards.