画蛇添足 [huà shé tiān zú]
ruin the effect by adding something superfluous
There was a noble in ancient Chu ,after the offer sacrifice,he give a pot of libate to the costumers who came to help the offer sacrifice.The each said "this wine to drink is not enough for everyone to have a drink.Let's draw snakes on the ground,who first painted completely should drink this wine."
One man finished his snake first.He is already to drink,but complacently hold the wine on his left hand,and used right hand to draw seet on the snake,said:"I'm able to give it a few feet more!" But before he finished painting the feet,another man has painted the snake.The man rffced the pot,said:"snakes have no feet,how can you give it some feet!" After said,he drank the wine.
The man add feet on the snake lose the mouth of wine at last.
认真翻译的,看看吧