Shortly after I started working for the Office of Telecommunications, my manager set up a demo that involved a minicomputer linked to a Votrax voice synthesizer. He had programmed it to say, "Hello! This is the computer talking ... ."
The temptation was too great. After he had left for the day, I studied the phoneme vocabulary for the Votrax, and replaced his program with one that started out: "Howdy, y'all!"
Dave was not too happy when I showed up the next day, and only slightly mollified when I showed him the backup copy I had made of his program. Practical jokes need to be carefully calibrated, I guess.