Silence of the fans
Oddly enough, the times my mistyping or forgetfulness has brought down kit, either the customers were quite happy to be told before they noticed, or the resulting loss of smoke was not expensive enough to worry about. Green fire yes, but nothing worse than that.
There was the night I arrived at work expecting to find that the flaky router we were trying to figure out had fallen over again, and tried to get on wireless. No luck. Very odd. New plan, prop open the door to the switch room and plug directly into the core - only the links to the server room in question were all dark.
The sensation of walking through what should have been an operating server room, alone and at night, with the bit of sand you have under your shoes crackling loudly, was kind of neat - though it took four hours I would rather have spent in bed to get the Aperiodic Pixie Converter with its failed bypass relays bypassed.