My short list
Edward II. Oman referred to Bannockburn as the worst defeat suffered by an English army. Losing with the dominant pike-and-longbow system to a smaller army is pretty bad.
Gen. Elphinstone, whose irresolution got an entire British army (save one) wiped out on the road from Kabul.
Santa Ana. Not masking the Alamo, getting surprised at San Jacinto and losing every battle he fought in 1847.
Tag team of Samsonov and Rennenkampf, losing at Tannenberg and the Masurian Lakes.
I have to concur on Burnside (Rohrbach (later Burnside's) Bridge at Antietam, Fredericksburg, the Mud March and the Crater at Petersburg).
Edward II. Oman referred to Bannockburn as the worst defeat suffered by an English army. Losing with the dominant pike-and-longbow system to a smaller army is pretty bad.
Gen. Elphinstone, whose irresolution got an entire British army (save one) wiped out on the road from Kabul.
Santa Ana. Not masking the Alamo, getting surprised at San Jacinto and losing every battle he fought in 1847.
Tag team of Samsonov and Rennenkampf, losing at Tannenberg and the Masurian Lakes.
I have to concur on Burnside (Rohrbach (later Burnside's) Bridge at Antietam, Fredericksburg, the Mud March and the Crater at Petersburg).