In the Jin Dynasty(265-420), a man called Yue Guang 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. Realising this, the friend recovered quickly.
This idiom indicates a condition of being over suspicious bringing trouble on oneself.