一顆老鼠屎壞了一鍋湯 [－颗－－－坏－－锅汤]

Jyutping jat1 fo2 lou5 syu2 si2 waai6 liu5 jat1 wo1 tong1
Pinyin yī kē lǎo shǔ shǐ huài le yī guō tāng

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