[user104658]

It may not even have been supposed to be in there - it could just have found it's way in. This is why there are products that say "Made in a factory that contains nuts" and things like that - because while the product itself doesn't have nuts in the ingredients, they can't guarantee that traces of nuts / nut oil won't have gotten in during the process, and some people have such a severe allergy that even a small trace is a problem.

Honest opinion? I would say that eating from a sandwich chain that makes various types of sandwich is ALWAYS going to be very risky for someone with a seed allergy... whether they have warnings up or not, I would just always stay on the side of caution and not do it. TBH if I had a sesame seed allergy I'd just avoid all bread products unless I had baked it myself or something.

Doesn't make the story any less tragic, of course, she was just a young girl and it's possible that the full extent of her allergy wasn't known until this reaction.