From On building systems that will fail (1991), by Fernando J. Corbató
- Imagine a UNIX system with one admin user.
- The system grows, and the single admin user become a bottleneck. So it's decided that multiple people can log into the admin user at the same time.
- A certain (unnamed) text editor is used to edit files. This text editor assumes that only one instance of it will be open at a time.
- One person is logged in as admin, and starts editing the system password file.
- Another person is logged in as admin, and starts editing the message of the day file.
- One ill-timed save file later, and the system password file is saved as the message of the day.