2 sentences

1. 由于他的疏忽，原本万无一失的计划最终出了岔子。

   yóu​yú tā de shū hū, yuán​běn wàn​wú​yī​shī de jì​huà zuì​zhōng chū le chà​zi.

   Due to his negligence, the plan that was supposed to be foolproof ended up hitting a snag.

   Source: Dong Chinese Dictionary

2. 为了确保计划万无一失，我们必须仔细检查每一个细节。

   wèi​le què​bǎo jì​huà wàn​wú​yī​shī, wǒ​men bì​xū zǐ​xì jiǎn​chá měi yī gè xì​jié.

   To ensure the plan is foolproof, we must carefully check every detail.

   Source: Dong Chinese Dictionary