Back when I worked for a small ISP, we had a diesel generator in case the power went out longer than our UPS batteries would last. This provided a great sense of security until we decided to test the system by powering off the main break and... it didn't start!

It turns out the emergency stop button was pushed in. Easy enough for us to fix then, but if the power had gone out at 4am it would have been quite another matter.

After that incident, we turned off the main breaker to the building weekly. It was great fun, as most of our offices were in the same building. We had complaints for the first couple of months until everyone got used to it and had installed mini UPS's for their office equipment.

We did actually have to use the generator for real a while later. Someone had driven their car into the local power substation, and it was at least a month until it was fixed. Electricity was restored through re-routing fairly quickly, but until the substation was repaired we were getting a reduced voltage that caused the UPSs to slowly drain...