Norms and nudges are both popular types of interventions. Recent years have seen the rise of "norm-nudges" - nudges whose mechanism of action relies on social norms, eliciting or changing social expectations. Norm-nudges can be powerful interventions, but they can easily fail to be effective and can even backfire unless they are designed with care. We highlight important considerations when designing norm-nudges and discuss a general model of social behavior based on expectations and conditional preferences. We present the results of several experiments where norm-nudging can backfire, and ways to avoid these negative outcomes.