A friend of my mother's is violently allergic to - of all things - potato. (Given that she's of my mum's generation, I'm surprised she's survived, because I suspect she had the allergy before anaphyllaxis was a word anyone knew).

Given that restaurants often use potato starch as a cheap way of thickening sauces - but deny it because it's not the classical way to thicken some sauces - this one can be a minefield. Potato also seems such a hugely improbably thing to be allergic to that I suspect she might have difficulty convincing people.