Battle of Britain. If the German command had stayed with the original objective of targeting Airfields and forcing the RAF into the open they would have won.
Hitler panicked at Bagration (and rightly so)... he pulled forces from a lot of different places to try to stop the bleeding... he couldn't use the 10 mobile divisions in the west because they where actively engaged trying to box in the normandy beachhead... if they where sitting around not doing anything he could be tempted to pull a corps or two out stating that he would transfer them back west as soon as the landings started or the eastern front stabilized
he pulled forces from the west (Leibstandarte, 9th and 10th ss divisions) to rescue the first panzer army at kaments even though he was on pins and needles about a potential landing in france in the spring of '44
I would contend that the Russian and Austrian armies are hardly unbeatable if Napoleon can avoid a battle with both at the same time through.