My small cock-up
Many years ago, back when I worked for a telco, I was given the job of replacing a noisy fan tray in the processor module of a NEAX61E-VS (very small) digital exchange, the purpose of which was to provide the 0800 service for a smallish country.
Unlike its big brother, the NEAX 61E, the VS version had both of the hot-standby processors in a single module, rather than two separate modules.
What wasn't made very clear in the procedural documentation was that the 50v power feed to processor modules was via relays which were held operated by a signal from the module's fan tray - obviously to shut the processor down should the fans ever fail. As it turned out, this was a fine concept for the 61E, but not so clever for the VS version.
So, having undone the retaining screws I carefully removed the fan tray, only to be greeted by a distinct lack of blinky lights on the processor consoles and the cards which filled the processor module.
By the time I worked through the convoluted boot process (by modern standards) and then loaded the core software followed by the latest data backup (all from tape cartridge) people had most certainly become aware of my activities.
No point in posting as an AC for this, it was all documented and acknowledged at the time.