万无一失 Fail-safe
Explanation
形容事情做得非常周到，没有一点疏漏，非常有把握。
It describes a situation that is very carefully planned, with no omissions, very sure.
Origin Story
战国时期，秦国名将白起率领秦军攻打赵国，赵国大将廉颇坚守长平，双方僵持不下。秦王派使者到赵国散布谣言，说白起害怕廉颇，想用计谋诱使赵王更换主将。赵王信以为真，将廉颇换下，改由赵括接替。赵括虽然精通兵法，但缺乏实战经验，在长平之战中，被白起率领的秦军打得大败，最终全军覆没。赵括本人也被射死，赵国从此一蹶不振。这个故事告诉我们，战争是一件严肃的事情，不能轻易相信谣言，更不能用没有实战经验的人来指挥军队，否则就会造成不可挽回的损失。
During the Warring States Period, Bai Qi, a famous general of the Qin State, led the Qin army to attack the State of Zhao. Lian Po, a general of the State of Zhao, defended Changping, and the two sides were deadlocked. The King of Qin sent a messenger to Zhao to spread rumors that Bai Qi was afraid of Lian Po and wanted to deceive him into changing his general. The King of Zhao believed these rumors and replaced Lian Po with Zhao Kuo. Although Zhao Kuo was well-versed in military strategy, he lacked practical experience. In the Battle of Changping, he was decisively defeated by Bai Qi and his army, and his entire army was wiped out. Zhao Kuo himself was shot dead, and the State of Zhao never recovered. This story teaches us that war is a serious matter, and that we should not easily believe rumors, nor should we put inexperienced people in command of the army, otherwise it will cause irreparable damage.
Usage
万无一失通常用于形容计划周密、准备充分，表示事情非常有把握，不会出现任何差错。
“万无一失” is often used to describe plans that are well-thought-out and thoroughly prepared, indicating that things are very certain and no mistakes will occur.
Examples
-
这次考试我准备充分，万无一失。
zhe ci kao shi wo zhun bei chong fen, wan wu yi shi.
I am well prepared for this exam, I will do well.
-
这个计划经过精心策划，万无一失。
zhe ge ji hua jing guo jing xin ce hua, wan wu yi shi.
This plan has been carefully planned, there will be no mistakes.
-
我们必须做好万无一失的准备，才能应对各种突发状况。
women bi xu zuo hao wan wu yi shi de zhun bei, cai neng ying dui ge zhong tu fa zhuang kuang.
We must be prepared for all contingencies in order to deal with any unforeseen circumstances.