One day, I get a call from one of the training rooms at my office that the power in that room doesn't work and their laptops (there were around 15 people in that room) wouldn't charge. I rush over to discover a couple of my co-workers scratching their heads around some extension outlets on the floor, that were "daisy-chained" between them. Now, this was a fairly large room and all the power outlets were on the walls of the room; that being said, it was fairly obvious when i entered the room that all the extensions were in the middle of the room, with no cable going to the side of the wall where the outlets were... The extensions were connected between them, but nobody bothered to check whether one of them was actually plugged in

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Also , one of the best stories from this category is one i've heard a couple of years back, and i thought i'd share it, although it didn't happen to me:
So, there was this server that rebooted every morning at around 5 A.M, day after day. The IT admin checked for software errors, checked for hardware errors, scheduled tasks, processes that might cause this to hang at that specific hour, every morning at 5 A.M. He called the hardware guys who didn't find any problems with it. Frustrated, he decided to wake up one morning and drive to work, to be in front of the computer when the unexpected shutdown occurs. Needless to say, at 5 A.M the cleaning woman would come into that room and unplug the server to plug in the vacuum cleaner, after which she would plug the computer back in. :P
P.S: Lol at no.3

Cracked me up