The water in the basins of a public swimming pool turned a yellow colour around 2 pm subsequent to an excessive chlorine injection (Cl2). This “colouring”, coupled with a strong Cl2 smell in the air, alert swimmers. When first responders arrived, no hazardous Cl2 concentrations were recorded in the air, but basin measurements were twice the normal level; 12 children, members of a club, and 2 adults feeling the ill effects of chlorine releases were still hospitalised for a few hours out of precaution. Fire-fighters ventilated the facility. The municipality drained the swimming pool and ordered the installations inspected before reopening. The next day, since analyses from the Regional Health Agency showed compliance with recommendations, the pool was allowed to resume activities. Human error was responsible for discharging Cl2; the injection pump had not been turned on that morning, and the Cl2 accumulating in the circuit was suddenly injected upon starting the pump.