A spoon used to dish tofu at a salad bar is moved into a tray of carrots. Someone with soy allergies takes some carrots and breaks out in hives because of the tofu the spoon had been used for previously.
A woman with wheat allergies orders a steak at a restaurant. She makes sure to get hers without any of the wheat-containing marinade, but has a severe asthma attack after she eats a few bites of the steak. The culprit? The communal grill on which all of the steaks --- her wheat-free steak and the steaks made with the marinade --- had been cooked.
A two-year-old girl who has never been fed peanuts experiences shortness of breath and swelling of her lips after eating bread and jelly at home. After allergy testing, she is diagnosed with a peanut allergy. It turns out that the same knife had often been used to make peanut butter and jelly sandwiches for an older sibling, and there was sufficient peanut protein in the jelly to sensitize the child to peanuts -- and to trigger reactions in the future if she were to eat from that jelly jar.
All of these hypothetical situations are everyday examples of cross-contamination, a major food safety concern for people with food allergies or intolerances. In general, cross-contamination occurs when an unsafe food taints a safe food by touching it or by touching the same surface. From a food allergy perspective, this occurs when a food that doesn't include any allergenic ingredients is tainted with allergens in preparation, cooking, storage, or service. It's the very real possibility of cross-contamination that is the reason for policies like hand washing before classroom entry in schools with severely allergic students, or separate tables for lunches with nut products. Cross-contamination is also the reason many manufacturers voluntarily include warnings about manufacturing lines or facilities using major allergens (there is, to date, no legal requirement for manufacturers to do so, nor do manufacturers tend to label the presence of food additives or non-"big eight" allergens in their factories or manufacturing lines).