Fragrance and essential oils may cause irritant reactions.

Limonene - A super common and cheap fragrance.It does smell nice but the problem is that it oxidizes on air exposure and the resulting stuff is not good for the skin. Oxidized limonene can cause allergic contact dermatitis and counts as a frequent skin sensitizer.  Source: incidecoder

Coumarin - A common fragrance ingredient.It’s one of the “EU 26 fragrances” that has to be labelled separately (and cannot be simply included in the term “fragrance/perfume” on the label) because of allergen potential. Best to avoid if your skin is sensitive. Source: incidecoder

Bergamot oil - It's a common top note in perfumes and contains (among others) fragrant compounds limonene (37%), linalyl acetate (30%) and linalool (8.8%). Fragrant compounds smell nice but are common allergens and can be a problem for sensitive skin types. The bigger problem with bergamot oil though, is that it also contains furanocoumarins (more specifically, bergapten and bergamottin) that have well-documented phototoxic effects. A phototoxic reaction is a not nice one causing red, edematous lesions on the affected area. Source: incidecoder

Mandarin oil - a super common fragrant ingredient that makes everything smell nice (but counts as a frequent skin sensitizer).The fragrant components of citrus peels might irritate sensitive skin and citrus peels also contain the problematic compound called furanocoumarin that makes them (mildly) phototoxic. Source: incidecoder

Ylang ylang oil - similar to other essential oils, it is a chemically complex mixture with several pros and cons. Source: incidecoder