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http://en.wikipedia.org/wiki/Exercise_Tiger

Friendly fire incident
The first practice assault took place on the morning of 27 April[7][8] and was marred by an incident involving friendly fire. H-hour was set for 7:30 am, and was to be preceded by a live firing exercise to acclimatize the troops to the sights, sounds and even smells of a naval bombardment. During the landing itself, live rounds were to be fired over the heads of the incoming troops by forces on land, for the same reason. This followed an order made by General Dwight D. Eisenhower, the Supreme Allied Commander, who felt that the men must be hardened by exposure to real battle conditions.[1] The British heavy cruiser HMS Hawkins was to shell the beach with live ammunition, from H-60 to H-30 (i.e. 6:30 to 7:00 am) giving the beachmasters half an hour to inspect the beach and declare it safe. Several of the landing ships for that morning were delayed, and the officer in charge decided to delay H-hour for 60 minutes, until 8:30. This message was received by Hawkins, but not by a number of the landing craft, with the result that troops were landing on the beach at the same time as the bombardment was taking place. British Marines on one vessel[clarification needed] recorded in its log book (the only log which has since been recovered from any of the boats) that men were being killed by friendly fire. "On the beaches they had a white tape line beyond which the Americans should not cross until the live firing had finished." But the American soldiers said they were going straight through the white tape line and getting blown up.[1]

On the day after the first practice assaults, early on the morning of 28 April, the exercise was blighted when a convoy of follow-up troops was attacked by nine German E-boats in Lyme Bay.
Of the two ships assigned to protect the convoy, only one was present. HMS Azalea, a corvette was leading the nine LSTs in a straight line, a formation which later drew criticism since it presented an easy target to the E-boats. The second ship which was supposed to be present, HMS Scimitar, a World War I destroyer, had been in collision with an LST, suffered structural damage and left the convoy to be repaired at Plymouth.[9] Because the LSTs and British naval headquarters were operating on different frequencies, the American forces did not know this.[1]
When other British ships sighted the E-boats earlier in the night[clarification needed] and told the corvette, its commander failed to inform the LST convoy, assuming incorrectly that they had already been told. British shore batteries defending Salcombe Harbour had seen silhouettes of the E-boats but had been instructed to hold fire so the Germans would not find that Salcombe was defended.[1]
The E-boats left Cherbourg on patrol the previous evening and did not encounter the Allied patrol lines either off Cherbourg or in the Channel[clarification needed]. They spotted the convoy (convoy "T-4"), eight LSTs carrying vehicles and combat engineers of the 1st Engineer Special Brigade with a single corvette as escort, and then attacked.[nb 1]
One transport (LST-507) caught fire and was abandoned. LST-531 sank shortly after being torpedoed while LST-289 was set on fire but eventually made it back to shore.[10] LST-511 was damaged by friendly fire. The remaining ships and their escort fired back and the E-boats made no more attacks. 638 servicemen were killed:[nb 2] 441 United States Army and 197 United States Navy personnel.[1] Many servicemen drowned in the cold sea while waiting to be rescued. Soldiers unused to being at sea panicked and put on their lifebelts incorrectly. In some cases this meant that when they jumped into the water, the weight of their combat packs flipped them onto their backs, pushing their heads underwater and drowning them. Dale Rodman, who travelled on LST-507, commented "The worst memory I have is setting off in the lifeboat away from the sinking ship and watching bodies float by."[6]