My Favorite? When the British at islanwanda allowed their troops to run out of ammo...and the zulus matched spears and shields against bayonets.
The british died in their little defensive squares...almost 1400 of them.
if I had to pick a second blunder...Id have to say the battle of the crater at St Petersburg, Virgina. Imagine packing that much gunpowder under a confederate position,,,blowing it up...and then leading your men down into a hole that they couldnt climb out of, but the confederates could shoot into.
Big mistake.