Not me this one, but a classic.
One Friday afternoon Telecoms tech was checking a remote unmanned exchange, one of the checks was to measure the levels on the analog multiplexer for the trunks to the main exchange, which acted as the brains for the dumb remote.
The procedure was to plug a 6.5 mm phone jack, attached to a large fixed meter into each channel at a time. Unfortunately, this chap grabbed the wrong hanging jack, this on having 50v exchange battery on it. He then proceeded to plug into each channel of the carrier system, and was mystified when there were no reading. As he plugged in the last channel, the exchange went totally silent. Whole exchange was down for 2 days.