A Woman who felt better when following a vegetarian diet, turned out to be sensitive to Isoleucine (this is an amino acid abundant in animal products).
A child with OCD behaviour, showed a magnesium allergy. We narrowed it down to Magnesium Citrate. Later we were able to locate research papers on the connection between OCD (and other health problems) and Magnesium.
A Lady who thought she was allergic to wine (sulphides) because of coughing episodes after drinking, it turns out to be an inability to absorb B12. (Later research shows that B12 deficiency is connected to chronic cough).