For me, it's avocados. I am highly allergic, the smallest amount of avocado or guacamole has me violently ill for days.
However, I have had many a friend or waiter raise an eyebrow when I say I'm allergic...leaving me with the impression they think it's a dislike. After one particularly horrible restaurant experience, my husband and I now start going into detail of the results of any amount of avocado/guacamole touching my food whenever we sense disbelief on the part of the server.
Anyone else have a similar experience?