Back in the very early 1980s, I was a young kid working part time at a computer store. But I knew our Zenith Z100 computer (an early MSDOS, but NOT PC compatible machine!) very well, I was the one that everyone else came to to ask questions of, and when customers had a question others didn't have an answer to, "call back Friday evening or Saturday and ask for Nick" was the answer.
Then we hired Bob. Bob was both a really good guy and really knowledgeable. He knew the Z100 almost as well as me. So in a bit of good-hearted fun, I figured I'd show I was just a bit better.
This was back in the MSDOS v1 days, editing the PROMPT variable wasn't an option. Zenith was a government contractor, so their MSDOS printed out all kinds of legalese. I reasoned, correctly, that all the strings the OS displayed had to exist in the binaries, and the ones I was after were in COMMAND.COM. Bob went on vacation, and left his favorite boot floppy at the store! My chance!
So, I used DEBUG to edit COMMAND.COM to change the Zenith copyright and legal notice to read, "Hi, Bob! Nick was here!"
I then changed "DIR" to "CAT" ("catalog" -- a command that was the equiv of DIR or Unix ls on another of our products). I then changed the message, "Bad command or file name" to "I don't want to!". Keep in mind this was all done one character at a time with an ASCII to Hex table sitting next to me (and on my time, not the company's dime).
So, Bob comes back, sits down in front of a Z100, boots his floppy.
"Hi, Bob. Nick was here!"
[Bob: "hahaha..that's good"]
I don't want to!
At this point, Bob is roaring with laughter. "How'd you do that?" And of course, I tell him. I showed him that everything was still 100% functional, then copied over an unaltered COMMAND.COM and put him back to work.