"I just learned last week that North Korea and South Korea are on the northeast side of China, right near Japan. I always thought they were southwest of China, far away from Japan (sort of in the area of Thailand). I would have bet money on it, but I was very wrong.
It's possible, assuming you're American, that you were confused because another "North/South" country in our history is near Thailand, that being Vietnam. It's no longer split like Korea but many people older than thirty still have North Vietnam/South Vietnam in their memory, and it also abuts China to its north, so the confusion is reasonable.