Paraben paradox is a phenomenon described by Fisher in 1973. A paraben-sensitive persons, who react to parabens on patch testing and develop a dermatitis when paraben-containing products are used on compromised skin, may continue to use paraben-containing products on other areas of the body that have intact uninvolved skin, without any adverse reactions.
Paraben(Photo credit: Wikipedia)
Recently I observed a similar phenomenon with Glycolic Acid. This patient is taking Glycolic Peels for acne scars. She did not develop any reaction during the patch testing or the first session. However she developed this reaction on the neck after her second session after 2-3 days.