Stick to rigid rules instead of taking circumstances into account
刻舟求剑
kèzhōuqiújiàn

It literally means "nick the boat to seek the sword (make a notch on the side of a moving boat to show where to look for the sword which has dropped overboard)."

The Story:
There was once a man who bought a sword he loved very much. One day while crossing a river in a boat, the sword fell into the water. Horrified, the man rushed to make a mark on the side of the boat where the sword had fallen. When his boat reached the shore, the man jumped into the water at the spot he had marked on the boat, trying in vain to retrieve his sword.