Operation Tiger

All Things Considered highlights the disastrous practice landing for D-Day, which occurred April 28, 1944:

Sixty-eight years ago today, the Allies launched a massive dress rehearsal for the invasion of Normandy — the famous D-Day landings that would happen five weeks later. But that rehearsal turned into one of the war's biggest fiascos.
It took place on Slapton Sands, a beach in southwestern England. British historian Giles Milton wrote about the rehearsal on his blog last week.
"The beaches there are long and they're wide, so it gave the soldiers plenty of opportunity to really experience what it was going to be like," Milton tells weekends on All Things Considered host Guy Raz. "The beaches in the west of England are almost identical to the beaches in Normandy."
The rehearsal was given the code name Operation Tiger. The plan: To get landing boats into the English Channel, then have them simulate a water landing on the beaches of the Devon coast. The man in charge was the great Allied commander Dwight D. Eisenhower.
"He wanted to put them out in the rough waters of the channels, have them shaken around, [exposed to] seasickness, everything else that soldiers are prone to," Milton says.

German E-Boats stumbled onto the exercise and torpedoed the LSTs, then the troops got hit by friendly fire.  In the end, 946 soldiers were killed.  The army kept the incident secret for almost 40 years.