I recently had an experience for the first time. I'm allergic to shellfish, but not had a problem before, including while on a previous cruise on the same line, by not ordering any food with shellfish--it's generally not an unlisted ingredient, unlike say, corn or milk. This time, despite telling the server, I still had a reaction--I suspect there was a language barrier, which I find very disconcerting. I didn't, of course, order anything explicitly containing shellfish, and never received an explanation of what went wrong.
Shellfish allergies are not uncommon--is it typical for ships to use shellfish in dishes where it is not listed as an ingredient? I'll never not notify in advance again, but I'm surprised that they would do this.