I've given this story here before...
But we had a DEC engineer turn out to upgrade a VAX 11/780, one of about half a dozen in the computer room on that floor (there were two floors full of VAX hardware).
He turned up, all the jobs and users were shunted across to the next machine in the line, the ops shut it down and the power off message appeared.
The DEC engineer went behind the wall of hardware that was the VAX and threw the power switch on the wall... It became quieter.
For a moment, as he re-appeared, the ops stared at him, stared at the console saying power off, stared at the engineer. Then the screaming started. From the next VAX in the line. Yes, he had thrown the wrong breaker switch and the VAX with the extra load and users had gone bye-byes.