Cao Xiu
Cao Rui's greatest military loss came in 228, when the Eastern Wu general Zhou Fang tricked Cao Rui's distant cousin and regional governor Cao Xiu into believing that he was ready to surrender his troops to Cao Wei, while instead laying a trap for Cao Xiu. Instead of realizing that it was indeed a trap, Cao Rui enthusiastically approved Cao Xiu's plan, and this led to a major military disaster, but Cao Xiu's forces were saved by Jia Kui from total annihilation.