That reminds me of another one...
In parts of NYC, there are noise abatement requirements, and so many places have their generators mounted on the roof -- it's cheap real-estate, the exhaust is easier, the noise issues are less, etc.
The generators usually have a smallish diesel tank, and then a much larger one in the basement (diesel is heavy)...
So, one of the buildings that I was in was really good about testing thier gensets - they'd do weekly tests (usually at night), and the generators always worked perfectly -- right up until the time that it was actually needed.
The generator fired up, the lights kept blinking, the disks kept spinning - but the transfer pump that pumped diesel from the basement to the roof was one of the few things that was not on the generator....
W