We flag ingredients as controversial when there's published research suggesting health concerns — not just internet rumor. Examples include BHA/BHT (carcinogenicity studies), carrageenan (intestinal inflammation), menadione (artificial vitamin K3, liver toxicity in animal studies), artificial colors (behavioral effects, banned in some jurisdictions), and propylene glycol (toxicity in cats). The research isn't always conclusive but is concerning enough that many premium brands exclude these.