Whether you believe it was friendly fire (an oxymoron if I ever heard one) or a German E-boat attack, let's not forget the soldiers who died on 27 April 1944 in a training exercise off the Dorset Coast as part of Exercise Tiger.
946 (number is disputed) US soldiers died
https://en.wikipedia.org/wiki/Exercise_Tiger
www.theguardian.com/uk/2004/may/16/military.usa
This book is on my wish list
http://www.amazon.com/The-Forgotten-.../dp/0747544670
There also a few more books on this exercise.