Nudges are popular types of interventions. Recent years have seen the rise of `norm-nudges' - nudges whose mechanism of action relies on social norms, inducing or changing social expectations. Norm-nudges can be powerful interventions, but they can also fail to be effective and even backfire unless they are designed with care. I highlight important considerations when designing norm-nudges and discuss a general model of social behavior based on social expectations and conditional preferences.