百不失一

bǎibùshīyī

(1) [cannot possibly go wrong; not a failure out of a hundred trials; never a failure in many trials] 一百次中也不会失误一次。 形容极有把握, 决不会出差错

从门应庭, 听堂室之言, 什而失九, 如升堂窥室, 百不失一。 --汉·王充《论衡》

(2) 又说"百无一失"