A North Korean soldier who defected to the South managed to scale three barbed wire fences along one of the world's most heavily patrolled borders without being detected.
Then he had to knock on barracks doors twice when he reached the South to get any attention.
News of the defection trickled out this week as opposition lawmakers in Seoul, who apparently learned of it through a leak, pressed the government to explain how such a security breach could have happened.
On Thursday, President Lee Myung-bak ordered that border guards in the area be disciplined.
The startling details of the guards' failure to spot the defector are proving an embarrassment for the government, with news media and politicians asking what might have happened if the soldier had been armed for an attack.
The North Korean soldier, whose name, rank and motive for defection were not disclosed, crossed the eastern border on the night of October 2. The military did not disclose his defection until lawmakers began asking questions about it during a parliamentary session this week.
In contrast, the military immediately revealed the defection of a North Korean soldier who crossed the border last Saturday after killing two of his officers.
South Korean guards spotted him and guided him into their side using a loudspeaker.
After the security breach was made public, Defence Minister Kim Kwan-jin spoke by teleconference with military commanders and said: "We are sorry we caused concern to the people."
Lee ordered "a thorough investigation and stern reprimand against those responsible" for the lax vigilance, his office said.
The North Korean soldier, attached to a military unit 50 kilometres behind the front line, reached the northern edge of the four-kilometre-wide demilitarised zone separating the two countries around 8pm.
The zone is guarded by sentries on both sides at night.
The North Korean side is in darkness, but the southern sector is lit with floodlights to help soldiers in towers spot intruders.
But the North Korean defector was not spotted as he scrambled over the tall fences, topped with concertina wire.
At around 11pm, he knocked on the door of a South Korean guard unit. When there was no response, he walked to another nearby barracks and knocked again. When South Korean soldiers answered, he turned himself in, military officials said.
Although thousands of North Koreans defect to the South though China, it is rare for a North Korean - soldier or civilian - to defect through the inter-Korean land border, which stretches about 260 kilometres west to east.