一粒老鼠屎壞了一鍋粥 [－－－－－坏－－锅－]

Jyutping jat1 lap1 lou5 syu2 si2 waai6 liu5 jat1 wo1 zuk1
Pinyin yī lì lǎo shǔ shǐ huài le yī guō zhōu

Definitions (CC-CEDICT)
  1. lit. one pellet of rat feces spoiled the whole pot of congee (idiom)
  2. fig. one bad apple can spoil the whole bunch