Daily Chinese Idiom Guessing Game
Absolutely certain, no risk of error or failure.
失差错。指非常有把握，绝对不会出差错。
《史记·淮队侯列传》以此参之，万不失一。”汉·枚乘《七发》孔老览观，孟子持筹而算之，万不失一。”
We checked the plan twice to make sure it was foolproof and wàn wú yī shī.