百日の説法屁一つ（ひゃくにちのせっぽうへひとつ）

Japanese vocabulary

Meaning: single mistake which ruins all one's hard work; one blunder rendering a long period of effort useless; one fart in a hundred days of sermons

See it used in real works

• Example sentences from Japanese novels & web fiction (Massif)
• Hear “百日の説法屁一つ” pronounced in real videos (YouGlish)