[Man50D]

My father participated in the practice landing. He vividly remembers seeing a huge plume of black smoke in the distance and thinking they were going to great lengths to make it seem real. He later found out it was real as a German PT boat had penetrated the practice area and torpedoed a ship. Several hundred military personnel were killed.

[Oatka]

He later found out it was real as a German PT boat had penetrated the practice area and torpedoed a ship. Several hundred military personnel were killed.
and
I don’t recall any details of when and where, but there was another landing exercise which went horribly wrong, drowning thousands.

I think you all are referring to the disaster at Slapton Sands, UK.

Others will no doubt join in. Here's a snippet from Disaster at Slapton Sands - april 27, 1944: Two WW II veterans and survivors of the disaster has recently been able to visit the spot in the Channel where 946 American servicemen, have died the 27th of april 1944 in Devon, at the south coast of England, in a last exercice for the preparation for the landings in Normandy.