The number 4
Similar to other East Asian cultures, the number 4 is considered unlucky in Korea because it sounds like the word for "death" in Sino-Korean pronunciation. It's common for buildings to skip or relabel a 4th floor (sometimes marked "F" instead), a detail visitors sometimes notice in elevators without immediately understanding why.
Writing names in red ink
Traditionally, writing a living person's name in red ink is associated with death (red ink was historically used to record the deceased) and is generally avoided, even today, in more formal or traditional contexts. It's a minor thing to know if you're ever writing a Korean colleague or friend's name on a card or document.
"Fan death"
A distinctly Korean urban legend holding that sleeping in a closed room with an electric fan running overnight can be dangerous. It has no scientific basis, but it's a well-known enough cultural touchstone that it sometimes comes up in conversation, especially in summer — worth knowing as context if a Korean friend mentions it, rather than a warning to take literally.
Why this is worth knowing at all
None of this changes how you should plan a trip, but recognizing these when they come up (an odd elevator floor label, a comment about fans) adds a small layer of understanding to details that would otherwise just seem random.