Synopsis: The Large Hadron Collider (LHC), the main particle accelerator at CERN, Geneva, is the largest and one of the most complex scientific instruments ever built. The accelerator depends on the reliable operation of strings of superconducting magnets cooled using helium at 1.9 K: at maximum current the total stored energy of the magnet system is about 13 GJ. For 20 years hundreds of scientists and technicians worked painstakingly on its design and construction. Resources were tight, and

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... ause of the unique nature of the project it is not surprising that a number of serious problems were experienced during construction and commissioning -problems that were nevertheless resolved as they appeared. It is therefore easy to imagine the disappointment caused by the severe incident that occurred just nine days after the spectacular start-up of the machine on September 10, 2008. The purpose of the present report is to examine the extent, the causes and the consequences of the incident, and to address the issue as to what can be done to reduce the likelihood of such incidents occurring in big science projects.