I'm not a historian, but I am an afictionado of D-Day stuff, and here's my two cents....
Having toured the beaches and museums of the Normandy coast, and from all the reading I've done over the past couple of years, here's what I think are the big mistakes the Germans made....
1) Hitler having control over the armor. If his subordinates had been able to call it in, it would definitely been a tougher, bloodier campaign.
1a) The subordinates being too chicken-sh*t to wake up Hitler. I'm pretty sure that if he was woken-up to be told that an Invasion was impending / underway, he wouldn't be all that angry about it....
2) Putting their faith in Calais and being fooled by Patton's "Fake Army". An ingenious idea by the Allies if I may say.
3) Not having sufficient forces defending the Atlantic Wall. Granted, commiting this many men (to stand on what could be described as Glorified Guard Duty) could be viewed as a waste of manpower... when that manpower could better be used in mobile fighting units.
One friend of mine, a fan of Axis armor, said that the Germans guaranteed a "non-win" of the war at Kursk. Personally, I don't know enough about it to be able to voice an opinion on that.
But I have heard that another mistake that the Germans made was fighting a war on two fronts. True? I don't honestly know... I'll leave that to the historians, and get back to my Model Bench.
Thanks fellas....
Paul.