The Herald reports that the town of Takaka was evacuated after a fire at a dairy factory.
Nelson fire officer Rob Allan was reported as saying: "The focus was on keeping the fire away from the silos of caustic soda and sulphuric acid, If they do combine under a fire situation the gas that's produced is mustard gas" .
Not to doubt him or anything, but when I was at school H2SO4 + 2NaOH => Na2SO4 + 2H2O
Sodium sulphate is a mostly harmless salt and I think you know what H2O is..
To make mustard gas you actually need ethene (ethylene) and sulphur dichloride - I can't think of a good reason to have those in a dairy factory - unless they are making some very odd flavoured yogurt? The only chemicals you might have around that are nasty when combined would be Sodium Hypochlorite (bleach) and any acid, e.g: (2NaOCl + 4HCl => 4NaCl +2H2O + Cl2). (You can try this in your own dunny if you want to expire from Chlorine poisoning).
Warning to kids of all ages: don't try this at home.
UPDATE: This article in the Herald suggests that the fire service and/or Fonterra's Health & Safety people are somewhat embarrassed by their error. Could I suggest chemistry lessons?