I seriously doubt it is a mistake. Blank arty rounds are clearly marked in blue. it is very hard to make a mistake. I think more likely the South is testing how fast and how the North will react to it. Considering they are changing their rules of engagement, they might simply be testing waters.
Those who were in arty for NS will know... such an "accident" is nearly impossible. The gun has to be maneuvered to face the right direction, the firing solution is calculated deliberately (c'mon 200m short on a 14km flight? 1.4% error lands it on the other side), amongst other things. No way it can be the error of a single "artillery man". If it's any accident, it's an "accidental message".
N.Korea Boosts Coastal Defenses as Allied War Games Start
North Korea has threatened to respond "mercilessly" to South Korean-U.S. military exercise that began Sunday.
In time with the start of the joint exercise, the North Korean military fired about 30 artillery shells from the Kaemori region north of Yeonpyeong Island into the West Sea as part of a military drill of its own. It also moved some of its 122 mm multiple rocket launchers and opened more camouflaged artillery gates to coastal positions.
The North moved SA-2 surface-to-air missiles with a range of about 30 km to the coast and placed surface-to-ship missiles with ranges between 83 and 100 km on launch pads on the western coast. Mig-23 fighters are on standby at Hwangju Air Base.
The Eighth Navy Combat Fleet in the West Sea and South Hwanghae Province are in a virtual state of war.