杯弓蛇影 (bēi gōng shé yǐng)
In the Jin Dynasty (jìn cháo 晋朝,265-420), a man called Yue Guang (yuè guǎng 乐广) once invited a friend to have a drink at his home. When the friend lifted his cup, he saw a small snake in the wine, yet he forced himself to drink. Back home, the friend recalled the incident, and felt so disgusted that he fell ill. Hearing about this, Yue Guang invited his friend again. He asked him to sit in the same place and drink. Then his friend saw that the image of the snake in the cup was actually the reflection of a bow hung on the wall. Realizing this, the friend recovered quickly.