How can I deliberately fail a mutation for testing purposes?

[john1625b]

For example in the following code, how do I make the code go into the .catch block

    changeSubjectLine({
      variables: {
        campaignId,
      },
    })
      .then(() => {
        toast(successToast('Subject line dynamic saved.'));
      })
      .catch((e) => {
        toast(errorToastConfig(e));
      // I want the code to go into here
      });

Is there a flag I can put in there to make it do that