A long time ago..
Windows 95 had the "It is now safe to turn off your computer" message on shutdown. This technician thought it was a witty jape to edit the image by copying the t of to and pasting it over the w of now, and saving the image on all the machines in they department computer lab.
None of the students noticed, and I felt a complete heel when a completely genial, but venerable, emeritus professor rushed up to me in a panic, apologising frantically because he thought he messed up my computer. I stopped witty japes shortly after.