In a refinery, a series of malfunctions on the catalytic cracking unit caused 10 tonnes of catalyst to be sent to the stack. At the beginning of the afternoon, an initial operating anomaly appeared: activation of a boiler involved in supplying air to the regenerator. No anomaly was detected inside the reactor. A half-hour later, a lower reaction temperature was recorded. Pressure at the level of the reactor and regenerator simultaneously dipped, while differential pressure at the cyclone separators increased. The high level alarm sounded on the tertiary cyclone. The catalyst level then dropped a number of times at the bottom of the stripper and regenerator. Control room operators used all variables available in order to restore a normal situation, but given the limited impact thus far decided to apply the emergency strategy developed subsequent to previous incidents and shut the unit down at 6:30 pm. An initial assessment indicated the discharge of 10 tonnes of catalyst to the stack The effort to normalise the situation had lost precious time and triggered a secondary discharge of catalyst to the stack. The refinery operator revised the facility’s inadequate operating guidelines and lowered the alarm threshold setting on the tertiary cyclone high-level detection.