In Korea, it's considered to rude when you smoke a cigarette in front of elderly people. Even if you are in smoking area, young people cannot smoke their cigarette face to face with elderly people. 

And when you drink with seniors, you have to fill up their glasses before it gets empty. Which means you are paying attention to that person. When you are not fill up the glass even if the glass has remain empty for some time, it considered to be bit rude. 

This is the thing that foreigners who visiting or living in Korea get easily considered as rude person when they don't know why they are considered to be the rude one. You MUST hand it over something with TWO hands. If receiver is younger than you, it's okay to hand over with one hand, but it's considered to be SUPER RUDE when you hand over something with just one hand to the seniors.