Once upon a time, the State of Chu planned a surprise night attack on the State of Song. The Yong River, however, posed a natural barrier.
To overcome this, Chu sent scouts to secretly measure the river's depth and place markers for wading across at night.
When night fell, the river's water level suddenly rose. Ignoring this change, the Chu army attempted to cross stealthily using the daytime markers. One thousand soldiers drowned. Desperate cries and chaos erupted on the river. Instantly, the army collapsed and was routed.