In search of a cappuccino, cheese bread, and a place to check her email, Silvia walked into a coffee shop. Upon connecting to the Wi-Fi hotspot, a captive portal prompted her to log in and offered a few third-party authentication options. When she clicked on one of the access token providers, her browser showed a "No Internet Connection" error. Since she didn't have access to the network, she could not get an OAuth token—and she couldn't access the network without one.
This anecdote illustrates a critical detail of system design that can easily go unnoticed until an outage takes place: cyclic dependencies.