South Korean troops failed to notice a North Korean man who crossed the heavily fortified border this month, even though he has appeared in front of television cameras eight times and raised alarms.
The South Korean military said it would take that moment to make the necessary changes.
The man swam from the sea in a wetsuit, walked 5 km and went three hours undetected before troops finally jumped into action in his ninth television appearance.
It is not known why he made the crossing on such a dangerous route.
At one point, the man walked through a drainage tunnel in the Demilitarized Zone (DMZ) that the South Korean military was unaware of even, the Yonhap News Agency reported.
This despite the fact that the area is one of the most heavily fortified and dominated by sensors in the world.
What happened during the trip?
The latest details come from a report by the South Korean Joint Chiefs of Staff (JCS) on the security failures on February 16. The JCS has refused to identify the man and is investigating whether he was trying to defect.
It is not known where his journey started, but he made it ashore in his wetsuit and fins at 1:05 am, near a South Korean observatory, north of the eastern border town of Goseong.
He hid the wetsuit and fins under a rock, then followed barbed wire south along the beach.
He then entered the drainage tunnel and entered the demilitarized zone.
The man then proceeded on an undetected road for more than 5 km (three miles) until 04:16.
His ninth appearance on a CCTV camera at this point led troops to report his presence.
The army found him at 7:27 a.m. The report said he was looking for civilians to surrender to because he feared the soldiers would take him back to the north.
What were the security gaps?
Surveillance cameras spotted the man several times between 1:05 am and 1:38 am, the alarms were triggered twice. No action was taken.
Military CCTV cameras caught the man three more times around 4 a.m.
A JCS official told Yonhap that the soldier in charge of coastal surveillance equipment was making adjustments at the time and believed the alarms were a system error.
Some questioned whether the man, a civilian working in the fishing industry, had swam from North Korea as claimed, given the cold winter conditions. There was speculation he could have done part of the boat trip instead.
However, the JCS said they believed the man swam, saying his wetsuit was in good condition and the tide would have helped.
This is the second time in four months that questions have been raised about security in the DMZ.
After a former gymnast defected last November by jumping over the border fence, the South Korean military said it would check all sensors.
The latest event led to an investigation of the 22nd Division, which is in charge of this area of the DMZ.
Are defections frequent?
Since coming to power in 2011, North Korean leader Kim Jong-un has reportedly ordered the strengthening of border controls between the two sides and with China, including by laying more landmines.
But around 1,000 people leave North Korea each year, fleeing a repressive state that has faced numerous charges of human rights violations.
Most North Koreans escape by crossing the border into China, where they risk being returned to the North.
Crossing via the DMZ is dangerous. If spotted and arrested by the North Korean military, those attempting to cross would likely be taken to a detention center for questioning. They could be tried and sentenced to long terms in the labor camps.
The border and its fortifications have been in place since the Korean War ended with an armistice in 1953. North Korea and South Korea remain technically at war because the fighting did not end in a peace treaty. .