The Domain Name System (DNS) is fundamental to the operation of the Internet. Failures within DNS can have a dramatic impact on the wider Internet, most notably preventing access to any services dependent on domain names (e.g. web, mobile apps). Although there have been several studies into DNS utilization, we argue that greater focus should be placed on understanding how and why DNS queries fail in-the-wild. In this paper, we perform the largest ever study into DNS activity, covering 3B queries. We find that 13.5% of DNS queries fail, and this leads us to explore the root causes. We observe significant differences between IPv4 and IPv6 lookups. A handful of domains that have high failure rates attract a huge volume of queries, and thus dominate the failures. This is particularly the case for domains that are classified as malicious. The success rates also vary greatly across resolvers due to the differences in the domains that they serve and the infrastructure reliability.