Inside a petrochemical unit, a steam supply problem encountered at the site’s steam production plant caused activation of the cracked gas compressor. The steam cracker was immediately shut down and the gases routed to the flare, resulting in the flaring of 800 tonnes of a hydrocarbon mix between Saturday evening and Sunday end of the afternoon. The unit’s supply was being provided by 2 boilers, one serving as a backup to the other. During the incident, one of the boilers was taken off-line for maintenance, leaving just a single boiler running. The idle boiler had undergone numerous safety tests, one of which called for closing the intake valve. The test operator mistakenly closed the fuel intake valve on the operating boiler from the control panel, causing a significant and sudden drop in steam supply to the units. With the steam cracker shutting down immediately, the installations were degassed and the flare network used as a backup for hydrocarbon ignition. To mitigate this type of error, the site operator improved boiler differentiation appearing on the control room’s graphic interfaces.