We had this test installation of an alarm system standing in a 19" rack.

Now and then in the morning when we came to work, the unit had restarted sometime before we got there. We spent a lot of time trying to figure out what errors we could have in our new software that made it reboot - the system did have huge amounts of configurations, and much things should happen depending on time of day etc. At one time it managed to run over the full weekend before failing.

Since we were busy testing new firmware, we did run the system with an EPROM emulator - an S4 programmer with a ribbon cable to the CPU board. We finally found that the lockup and watchdog reset came when the first person came to work and turned on the corridor lamps. We didn't think about such problems earlier because we could operate much other equipment without upsetting the system.