Yes, you have a breach of warranty claim. Much stronger, of course, if you have disinterested witnesses to your having emphasized your shellfish allergy to the waiter, and if you have proof that the taco contained shellfish. Submit your ambulance and ER/hospital bills with your claim. I recommend using a lawyer. The restaurant's insurance carrier may ignore your claim, and will almost certainly offer you less in settlement, if you don't have a lawyer.
Answered on Sep 19th, 2012 at 12:58 PM