Today is the 75th anniversary of "Exercise Tiger" the preparation for the D-Day invasion. 750 American soldiers died when German E-Boats patrolling in the English Channel came across the practice maneuvers and opened fire.
https://www.historyandheadlines.com/april-28-1944-exercise-tiger-d-day-practice-kills-americans-d-day/
https://www.historyandheadlines.com/april-28-1944-exercise-tiger-d-day-practice-kills-americans-d-day/