A colleague phoned a school in the wilds of Scotland to diagnose an IT fault and wanted the guy to reset the modem. "Turn the power off, count 5 then turn it on again." The guy puts the phone down then my colleague heard a distant voice "but he told me to turn it off." Luckily it was a small one-room school!
Once worked in a media broadcast centre where everything ran off DC. We got a job to check the float battery was OK. This was a big room filled with bathtub sized lead-acid cells and to check each was giving 2V we first had to isolate the battery from the rectifier unit in a room across the corridor. We had told everyone what we were going to be doing and to ignore the big blue POWER FAIL light... well we thought we had. We had checked a couple of cells when my mate got that "Oh $#!t!" feelings... he rushed across the corridor to discover the rectifier had been tripped! The one guy we hadn't spoken to had seen the alarm, got to the rectifier and, in a panic, pressed everything in site... including the one that took the rectifier offline. At the 'inquest' they said the power had been off for 27s... another 3s and the mega-bucks compensation clause kicked in!
Then there was the day that London Electricity managed to connect 2 phases across live/neutral and we had to go up and down Edgware Rd with the petty cash buying up all their stocks of 20mm fuses