Goofs

Capt. Jang and his men commandeer a North Korean Self-Propelled Gun (SPG) and use it to assault North Korean beach defenses at Wolmi-Do. However, its appearance does not resemble the actual North Korean SPG in service at the time, the SU-76. The real SU-76 had a thinner, longer gun on a rear-mounted casemate, while the one in the film has a front-mounted casemate housing a shorter, larger main gun.

The three corpses thrown from the train to the river (00:05:16) are falling down vertically when, due to the train speed, they must describe a parabola.

Actually combat in Seoul continued until September 28, although General Almond declared the city taken on September 25. Given September 15 as the first introduction of UN/American combatants at Incheon, that makes the 13 days accurate.

Onscreen text just before the credits tells the audience that "13 days after the successful landing operation of Incheon, on September 15, 1950, the UN Allied Forces reclaimed Seoul." However, the actual Battle of Inchon took place on September 19, 1950, and Seoul was not completely recaptured until September 25, 1950.

This is not entirely correct. UN forces began landing on September 15th. Still 2 days short of of the "13 days" claimed in the onscreen text, but not as off as claimed above.

One of the South Korean spies, when riding the back of a truck, to prevent enemies from stopping them, pulls a grenade pin with his teeth. He could have used is other hand to do it instead, as it usually takes at least 10 pounds (4 kilos) of force to remove it. Although it looks cool, it's also unrealistic to do it that way.

The wrist watch that military gives to the barber -around minute 16- has "QUARTZ" printed in the dial.

During briefing at 7:20 they show a kodak carousel slide projector. A quick google search found that those werent patented until 1965.

The moment a substitution was suspected, the imposter could have been quizzed by those who sent him. They would ask about things he would know nothing off.