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Exercise Tiger, or Operation Tiger, was the code name for one in a series of large-scale rehearsals for the D-Day invasion of Normandy, which took place in April 1944 on Slapton Sands in Devon. Coordination and communication problems resulted in friendly fire deaths during the exercise, and an Allied convoy positioning itself for the landing was attacked by E-boats of Nazi Germany's Kriegsmarine, resulting in the deaths of at least 749 American servicemen. Because of the impending invasion of Normandy, the incident was under the strictest secrecy at the time and was only nominally reported afterwards.
https://en.wikipedia.org/wiki/Exercise_Tiger