In a post-truth world, software doesn’t have to be malicious to be dangerous. For example, in 2016, Facebook replaced its (human) news editors with an algorithm. Within days, conspiracy theories were at the top of the trending feeds, presented as 'news.' In this talk, we'll discuss the ethical considerations of building software in a world that looks more like an unfolding episode of 'Black Mirror' with every passing week. No software exists in a vacuum. As developers, we have to understand the social implications of our software to help us make ethical decisions about what we get involved in, and how we build software in general. And we have to be brave and speak up when we think something could go wrong. And, if someone else speaks up, we must be willing to listen. In short: we must be humble, we must be watchful, and we must ensure the decisions we make mean we can sleep at night.