This document discusses various biometric and physiological measurement techniques that can provide hidden insights into users, including eye tracking, biometrics, electroencephalography (EEG), emotion recognition software, and measures of heart rate, galvanic skin response, respiration, and electromyography. It notes that these techniques aim to optimize user experiences and interactions while providing commercial advantages by unlocking insights from both conscious and subconscious human responses and behaviors. The document raises the question of which, if any, of these techniques should be used to test user experiences and considers what could potentially go wrong when applying these methods.