Strategypage reports that there have been multiple incidents where nerve gas has been detected along the Chinese-North Korean border:
Anonymous Chinese military sources revealed that, a year ago, nerve gas detectors on the North Korean border went off. Further investigation revealed small amounts of Sarin nerve gas. There were no casualties, but the detectors went off again three months later. The Chinese don’t have nerve gas detectors deployed on the North Korean border, but they do periodically send special operations troops to the border to check security, and these units carry the detectors with them.
The North Koreans denied any responsibility, but it’s long been suspected that a chemical plant in the North Korean town of Sinuiju (on the Chinese border and the location of one of the major bridges, on the Yalu river, connecting the two countries) produced nerve gas, along with non-military products. Apparently there were problems in the plant, but there were no reports (not unusual in North Korea) of any nerve gas casualties in, or near, the plant.