It was not quite a "war mistake" but it did start a war, and that was when the driver of Austrian Archduke Franz Ferdinand took a wrong turn down a street in Sarajevo, Bosnia, where an enemy of Austria seized the opportunity to assassinate the Archduke, an act that led to World War One.
The order the Afrika Korps recived at El Alemain, to hold their ground and fight to the last man. Destroyed the Afrika Korps that battle.
Thats what you get with a Totalitarian regime, some lunatic in charge who reckons he knows everything.
Little wonder Rommel why joined the Conspiracy against Hitler.
Union Major General John Sedgwick at Spotsylvania as he tried to motivate soldiers of an artillery battery who were pinned down by Confederate sniper fire. He was shot in the head and killed as he was speaking.