This actually happens more than you would think but it’s always nerve racking. From Alertnet :

SEOUL, Jan 27 (Reuters) – South Korea has returned fire after North Korea shot several artillery rounds into waters near a disputed sea border with the South on Wednesday, Yonhap news agency reported an unnamed military source as saying.

North Korea on Tuesday declared a no-sail zone in the waters off its west coast, according to media reports in the South.

Via N.T.A. who is keeping track of this story.