And if we're talking about Civil War blunders, the
Battle of the Crater has to be near the top of the list.
Summary of the battle: In the spring of 1864, Grant's army was chasing Lee through Virginia. The Confederates won tactical battlefield victories at Wilderness, Spotsylvania, and Cold Harbor, but this was all part of Grant's strategy to win the war through attrition: the Union could replace lost men and materials at a much faster rate then the Confederates could. Grant continually maneuvered his army to try to outflank Lee and threaten the Confederate capital of Richmond, and Lee was eventually forced to dig in at Petersburg, a vital rail hub. The ensuing Siege of Petersburg lasted for nine months and was an early preview of the immobile trench warfare that would become infamous on WWI's Western Front 50 years later.
To break the stalemate, a Union engineer devised a plan to dig a mine underneath the Confederate fortifications, pack it with explosives, and then blow a huge hole in the rebel lines. Union troops would then use the ensuing chaos to rush through the breach and make rapid territory gains. A unit of African American soldiers was trained specifically for this assault, but at the last minute, General Meade replaced the black troops with white soldiers who had not been trained for this task (he feared that if the assault failed, the Confederates would murder the African Americans instead of taking them prisoner, which would cause political blowback against Lincoln in the North just a few months before the 1864 presidential election).
The mine explosion went off exactly as designed, and a huge hole was blown in the Confederate lines. However, the replacement soldiers weren't familiar with the assault plan. Instead of attacking
around the crater, they stormed
into it, where they were picked off like fish in a barrel by rebel troops as soon as they recovered from the initial shock of the explosion. Union casualties were 3,800, compared to only 1,400 for the Confederates. The Siege of Petersburg would continue for another
eight months. General Grant later said of the debacle, "It was the saddest affair I have witnessed in this war."
The crater is still visible to this day.