大意失荊州

Etymology
From the Romance of the Three Kingdoms. Guan Yu had been assigned to defend Jingzhou. In a temporary lapse of judgement, he decided to go on the attack against Cao Cao. In doing so, he unwittingly gave Sun Quan an opportunity to attack Jingzhou. Guan Yu was captured, and later executed.

Idiom

 * 1) to have a temporary lapse in judgement that results in a major defeat; make a bad call and lose the farm