We present NEL (Network Error Logging), a planet-scale, client-side, network reliability measurement system. NEL is implemented in Chrome and has been proposed as a new W3C standard, letting any web site operator collect reports of clients’ successful and failed requests to their sites. These reports are similar to web server logs, but include information about failed requests that never reach serving infrastructure. Reports are uploaded via redundant failover paths, reducing the likelihood of shared-fate failures of report uploads. We have designed NEL such that service providers can glean no additional information about users or their behavior compared to what services already have visibility into during normal operation. Since 2014, NEL has been invaluable in monitoring all of Google’s domains, allowing us to detect and investigate instances of DNS hijacking, BGP route leaks, protocol deployment bugs, and other problems where packets might never reach our servers. This paper presents the design of NEL, case studies of real outages, and deployment lessons for other operators who choose to use NEL to monitor their traffic.