open a can of worms

To open a can of worms means to create a situation that will cause trouble or be unpleasant for other people. Usually, someone will say you opened a can of worms when you intend to say something or do something which you know ahead of time will cause problems or trouble for some of the people involved, but for whatever reason, you feel it is necessary. It may be telling a secret, revealing something that people arent aware of, or do something unpleasant.