It's okay. This happens to all major militaries at one point or another.

While conducting joint naval exercises in the Yellow Sea not far from where North Korea torpedoed the Ch'ŏnan and then shelled a town on a South Korean island, a South Korean army unit on the mainland accidentally fired a shell, which (thankfully) landed on the ROK's side of the DMZ.

From CNN:

The South Korean military accidentally fired a shell during a land-based military exercise Sunday afternoon, a South Korean military spokesman told CNN.

The country's defense ministry declined to provide details about the land exercise.

The United States and South Korea also began naval drills in the Yellow Sea Sunday, but a U.S. military spokesman has said no live firing will take place during those exercises.

The shell was fired by a unit located near Munsan, South Korea, and landed on the southern side of the military demarcation line, the South Korean officer said.

"The South notified North Korea that this was accidental firing through a statement issued by the chief delegate of inter-Korean general level talks," the officer said. He added, "the military is looking into the cause of the accident."

Yonhap also has the story. I say we "accidentally" fire loads more shells. Then if we ever do need to actually shell North Korea, they'll be caught off guard. After all, with their repeated shelling of the waters on either side of the NLL over the past year, that's what North Korea has been doing.

Meanwhile, Reuters has a nifty information-laden Q&A article on the joint military drills.