During the Battle of Antietam in 1862, he ordered his Union forces to cross the Potomac River in order to engage the Confederate forces in battle. They correspondingly marched across the bridge two abreast, making an ideal target for Confederate gunners flanking the bridge. Not surprisingly the slaughter was appalling.
Alas, Burnside had failed to notice that the river was only waist deep, and could easily have been crossed almost anywhere else in perfect safety.
[Burnside was famed for his heavy side-whiskers, which were soon given the apt nickname 'sideburns'.]