In the Warring States Period (zhàn guó 战国,475-221BC), a man in the State of Chu (chǔ guǒ楚国) was offering a sacrifice to his ancestors. After the ceremony, the man gave a beaker 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.

战国时代有个楚国人祭他的祖先。仪式结束后，他拿出一壶酒赏给手下的几个人。大家商量说：“我们都来画蛇，谁先画好谁就喝这壶酒。”其中有一个人先画好 了。但他看到同伴还没有画完，就又给蛇添上了脚。这时，另一个人也画好了，夺过酒壶吧酒喝了，并且说：“蛇本来是没有脚的，你怎么能给它添上脚呢？”

“画蛇添足”这个成语比喻做了多余而不恰当的事，反而把事情弄糟了。
 

画蛇添足(huà shé tiān zú)
【翻译】Drawing a snake and Adding Feet.
【释义】画蛇时给蛇添上脚。比喻做了多余的事，非但无益，反而不合适。也比喻虚构事实，无中
            生有。
【例句】本来很好的一个方案，经理还要让我们补充一些无用的环节，这简直是画蛇添足啊！
【近义词】徒劳无功、多此一举
【反义词】画龙点睛、恰如其分