The wikipedia has it partly wrong. The US sent a shipment of mustard gas munitions to the Med to be used in retaliation, if the Germans used chemical weapons in Italy. One hundred tons of mustard gas was loaded on the Liberty ship “John Harvey” that was in Bari Harbor. On the night of Dec 2, 1943, in an airaid by the Luftwaffe, the ship was caught fire from the flames of the ship moored next to it that had been hit. The fire caused the gas to be released in the harbor area. Because it was shipped in secret, no one in the harbor, except possibly the senior military official in Bari. This resulted in the medics not knowing what was the cause of the problems with the many sailors and people who were exposed to the gas and thus not giving them proper treatment.
Here is a much better and accurate article on the incident: http://fhpr.osd.mil/CBexposures/ww2mustard.jsp
Miraculously the attack was beaten off and none of the ships were hit. But if 1 had been hit the entire row would have gone up in a series of enormous explosions.