The One You Didn't Know About:

In April of 1944, preparations were in full swing for the upcoming invasion in June, and they figured the best way to ensure victory was to do a full-scale dry run so that the Allied soldiers could get a taste of the horrors they were about to face before the actual horrors started. Operation Tiger was a top-secret invasion rehearsal at a place called Devon, chosen for its resemblance to the French beaches, and kept way under wraps so that the Germans wouldn't know what they were rehearsing for. So imagine the Brits' surprise when the Nazis actually showed up.

Nine German E-boats managed to sneak through the defensive perimeter. Probably not realizing what they had stumbled into, they were presumably confused about why the Allied forces were storming a British beach. But they shrugged their shoulders and opened fire, sinking a bunch of Allied carriers who had mere seconds to realize this shit just got terrifyingly real.

One of the Sherman tanks from the actual practice run now stands as a memorial.

The Allies started desperately calling for help over the radios, but due to the secrecy of the mission, no one on the mainland knew what was going on, and due to a paperwork error, Royal Navy rescue ships were operating on a different frequency. What resulted was 946 American fatalities -- over three times the amount who died at Utah Beach in the D-Day invasion.

As you can imagine, suffering such a huge defeat in the practice run of the invasion didn't set very high hopes for the actual invasion, which still happened as planned 40 days later, with many of the survivors taking part. But historians believe that if it wasn't for Operation Tiger, the D-Day invasion may not have succeeded, because the Allies were forced to learn the hard way about whatever they got wrong the first time.

Via Wikipedia
Oh, that tank above? It was under water for 40 years.

This basically means that, by crashing the rehearsal, the Nazis ensured their own defeat in a frenzy of Darwinian adaptation. They were probably kicking themselves over that one.