We explore the relationship between norm-uncertainty and lying. Lies are ubiquitous, and people often lie for their own benefit or for the benefit of others. Research in environments in which social norms are clearly defined and communicated finds that social norms influence personal decisions, even when they are not in our own self-interest. We deviate from this approach and study lying under norm uncertainty with scope for opportunistic interpretation of the norm. We introduce variation along three dimensions: salience of different types of norm-uncertainty (normative/empirical), the beneficiary of the lie (self/other), and ex-ante knowledge about the opportunity to tell a lie in order to tease out potential belief-distortion mechanisms. We also find compelling evidence that individuals engage in self-serving belief distortion to increase lying overall. However, we observe this only when uncertainty about what others do (empirical uncertainty), but not when uncertainty about what others approve of (normative uncertainty) is made salient. We also observe conditional liars, but only when the lie is self-serving rather than to the benefit of a third party. We discuss policy implications to improve the effectiveness of norm-based interventions.