Foolish

[Duke Malcolm]

Prince Charles Edward Stuart/Stewart, Battle of Culloden.
After marching the Jacobite Army down to Derby, he then marched them back up to Inverness, because the Government's forces were closing in on them, even though a French force was waiting to assist him.
On arrival at Inverness, the army marched to a government base near Inverness to attack them by surprise, but dawn was breaking, and the enemy army was up, so the Jacobites had been marching all night without rest.
The Jacobite army marched back to Inverness, well, Culloden field. They set up with a wall to their right flank, and a bog to the front. The government army, led by the Duke of Cumberland, attacked them. The jacobite highlanders charged across the bog, which slowed them down, making them an easy target for enemy cannon, grapeshot, and musket fire. Enemy dragoons and musketeers set up along the wall, out-flanking the jacobites.

[AlexPeters]

Friedrich III. of Prussia and his glorious army of Jena and Auerstedt, more men, good discipline but he had choosen the oldest gernerals he could find in his country to command them. The youngest was about 65 i think. Instead of delaying the french armies they took them head on while a russian army was already on their way to support the prussians.
The commanders of the prussian fortresses in the same war are also top on the list, the garrison of the strongest prussian fortress Erfurt surrendered immediatly with 20000 men and 200 cannons because 20 french hussars were on the gates, if i'm remembering correctly.

Another one but i think it has been bad luck only:
The Admiral of the dutch fleet that had to surrender with his whole fleet because french Hussars had surrounded his ships while they were anchoring in friendly waters. His mistake, well, the weather was against him in this winter, his ships were enclosed by ice and the hussars rode over the frozen sea.

[RabidGibbon]

What about Raglan at Balaclava?

The orders may have been spectacularly carried out by the light brigade but that doesn't mean they should have been given, or they should have been clearer about which guns he mean't.

Or maybe Cardigan was to blame for following the orders.

Either way someone was pretty damn foolish.

[Duke Malcolm]

No, the Cardigan mis-interpreted the orders, because of bad grammar.
The orders were intended to move the Light Brigade down one valley, but Cardigan rode down the Valley of Death, because of bad grammar, and not pointing out which guns to go to.