Beating the Grass and Flushing Out the Snake

In ancient times there was a county magistrate who took bribes and practised graft. One day, somebody sent him a petition accusing his secretary of practising graft and taking bribes. The magistrate trembled when he read the petition. He wrote on it: 'You have beaten the grass and frightened a snake.' 

古时候有一个县官，贪污受贿，谋取私利。有一次，有人写了一个状子，控告他的秘书贪污受贿。这个县官一边看状子，一边直打寒颤。他在状子后面写了八个字：“汝虽打草，吾已惊蛇。”（“你虽然打的是地上的草，但我就像伏在草里面的蛇，已经受到惊吓了。”） 

This idiom refers to alerting the target of one's scheme by being incautious. 

这个成语比喻做机密的事行动不小心，使对方觉察后有所防备。