百密一疏 A hundred precautions fail for one oversight
Explanation
指事情考虑得很周密，但仍有疏忽的地方。
Refers to something that was very carefully considered, but still contains a small error.
Origin Story
话说当年秦始皇统一六国后，为了巩固统治，下令修建万里长城。这项工程浩大复杂，需要无数人力物力，秦始皇派大将蒙恬负责。蒙恬深知长城的重要性，事无巨细，都亲自过问，可谓百密。他调集了全国各地的能工巧匠，设计图纸精益求精，材料选择更是严格考究。然而，即使如此严谨的准备，在长城修建的过程中，还是出现了意外。由于负责运输粮食的官员贪污受贿，导致部分路段的工程队因为缺粮而停工，最终导致长城在某处出现了一段明显的缺口。这便是百密一疏，再缜密的计划也可能因为一个细微的疏忽而功亏一篑。
It is said that after Emperor Qin Shi Huang unified the six kingdoms, in order to consolidate his rule, he ordered the construction of the Great Wall. This huge and complex project required countless manpower and material resources, and Emperor Qin Shi Huang appointed General Meng Tian to be in charge. Meng Tian was well aware of the importance of the Great Wall and personally oversaw every detail, so it can be said that he paid attention to a hundred details. He mobilized skilled craftsmen from all over the country, the blueprints were refined over and over again, and the material selection was strictly scrutinized. However, even with such careful preparation, unexpected incidents occurred during the construction of the Great Wall. Due to corruption among the officials responsible for transporting food, some construction teams had to stop work due to food shortages, which eventually resulted in a significant gap in the Great Wall at one point. This is the meaning of "a hundred precautions fail for one oversight," showing how even the most carefully thought-out plans can fail because of a minor oversight.
Usage
用于比喻事情考虑得很周密，但仍然存在疏忽。
Used to describe that something was very carefully considered, but still contains a small error.
Examples
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他做事一向谨慎，但这次却百密一疏，犯了一个低级错误。
ta zuòshì yīxiàng jǐnshèn, dàn zhè cì què bǎi mì yī shū, fàn le yīgè dījí cuòwù
He is always cautious in his work, but this time he made a mistake because of carelessness.
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计划周全，但还是百密一疏，出了点意外
jìhuà zhōuquán, dàn háishì bǎi mì yī shū, chūle diǎn yìwài
The plan was well thought out, but there was still a small mistake, a small surprise, nevertheless.