麻繩專挑細處斷 [－绳专－细处断]

Jyutping maa4 sing4 zyun1 tiu1 sai3 cyu3 dyun6
Pinyin má shéng zhuān tiāo xì chù duàn

Definitions (Wiktionary)
  1. proverb
    The most critical person/step is the one most subject to flaws.