The most comprehensive analysis of root causes of HC leaks (above 0.1 kg/s initial leka rate) to date has revealed that almost one third of the leaks during the last 7 years occurred in association with preventive maintenance. This is a paradox, as the main purpose of preventive maintenance is risk reduction. Maloperation of valves during isolation work is the main cause in the last years. Break-down of isolation systems during maintenance is also a frequent cause. Failure to perform verification is the most frequent contributing factor. The full paper may be found here.