Ooops....
A few years ago I was replacing an old server we have co-located in a server farm. I was escorted to the rack where our server was located, and the engineer unlocked the cabinet and left me to work. I duly powered our server down, and went to the rear of the rack to remove the cables so I could slide the server out. I moved back to the front of the rack, started pulling on our server but it would not slide out, the cables were all still plugged in.
The cables I'd removed? Belonged to someone else's server. I quickly plugged everything back in and powered up the server, hoping that whoever it belonged to would not notice. I carried on with our server replacement, and no-one collared me when I left, so I did get away with it.