失常 describes a state where someone or something deviates from the normal or expected standard, often used for performance, behavior, or mental state. It's more specific than 'abnormal' — it implies that the person or thing usually performs well but has temporarily fallen short. You'll hear it in sports (表现失常 'play below par'), psychology (精神失常 'mentally disturbed'), and technical contexts (机器失常 'malfunction').