Mustard allergy is reported relatively rarely but is a growing concern, at least in Europe. According to a 2003 editorial in the European Journal of Allergy and Clinical Immunology, mustard was the fourth most prevalent food allergen in children, behind eggs, peanuts, and milk.
Boston Globe has recently published a case of a 3-year-old girl who is allergic to mustard -- read more on AllergyNotes.
Mind map of food allergy. Source: Food Allergy: A Short Review, AllergyCases.org.
Image source: Wikipedia, GNU Free Documentation License.