刻舟求剑
A man of Chu State was once taking a boat across a river when he carelessly dropped his sword into the river. He made a mark on the side of the boat where he dropped the sword, hoping to find it again later on. When the boat reached the other side of the river, he got off and went into the water from where he made the mark, in search of the sword. Naturally he couldn't find it in the end.