Personally I’ve only ever written two. One that created multiple textfiles in various folders under windows that all grew exponentially. This was in the age of HDD so it could actually caused some serious problems. I created it because a guy in my class stole my homework by copying my files. So i created this virus, named it something fitting and “forgot” to close my laptop when i went to the bathroom.
And another that once every 40 minutes + (5 to 50) minutes would open your cd tray and then close it after 2 seconds. No one used CD’s at this point and i just wanted to prank some colleges.
Do social engineering hacks count?
Back when I had a high-level tech job (for which I was severely underqualified), my team were given a decommissioned Linux server each to mess around with and develop on. I convinced one of my colleagues to let me log in to his server as root for a sec. Then I ran a quick shell one-liner that repeatedly kicked anyone that wasn’t on my tty. He could log in, only to be logged right back out again.
Prank successful, I cancelled it fairly quickly.
Then I let him have a turn on my server. Gave him the username “root” and its password. He got a fake shell that kind of, sort of, worked like Bash, but he couldn’t do much of anything.
If the quotes around “root” didn’t give it away, I’d changed the uid 0 username to something else and “root” was just a regular user. (And a couple of the fake shell’s commands would lie and say it was uid 0.)