画蛇添足

In the Warring States Period, a man in the State of Chu was offering a sacrifice（祭祀，供奉） to his ancestors. After the ceremony, the man gave a beaker（烧杯，大口杯） of wine to his servants. The servants thought that there was not enough wine for all them, and decided to each draw a picture of a snake; the one who finished the picture first would get the wine. One of them drew very rapidly. Seeing that the others were still busy drawing, he added feet to the snake. At this moment another man finished, snatched the beaker and drank the wine, saying, 'A snake doesn't have feet. How can you add feet to a snake? '

This idiom refers to ruining a venture by doing unnecessary and surplus things.