Well, there is precedent for such high levels of tension, but you have to go all the way back to Kim il-Sung and the DMZ conflict of the 60s, long before things like the Sunshine Policy. That included the incident where 31 men from North Korea were sent across the DMZ to assassinate the President of South Korea which ended in a running battle outside the Blue House (for analogy, imagine a firefight right outside the White House or Westminster Palace), an attack by the North Koreans on an American spy ship, the hijacking of a passenger jet liner, assorted kidnappings, and several other more "minor" attempts at infiltration, attacks on patrols or other teams of soldiers sent out (engineers on construction duties or the like), or assaults on vulnerable fixed positions. You are right, though, in that it's been a while, and it's not entirely precedented. Kim il-Sung usually wasn't this indirect (the 2010 Yeonpyeong incident probably wouldn't have just been a matter of shelling the island, but actually landing fire teams), and Kim Jong-il was much softer; the 1999 naval engagement at Yeonpyeong could probably be attributed more to DPRK sailors exceeding their mandate to fire their guns in what had to date been a game of bumper cars conducted with corvettes and torpedo boats.