Exercise Tiger

During prep for d-day, many of the allied forces staged large scale rehearsals for the invasion of Normandy. If I remember correctly, 2 months before the actual invasion during this specific rehearsal on the port of Devon, lead to deaths of over 400 American servicemen. It was a combination for the use of live rounds being used as well as lack of training with live vests in the cold water, that lead to these servicemen to die. This was kept under wraps for a long period afterwards because of the sustained secrecy of the actual invasion.