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.
It’s the only one and it was accidental. Don’t remember what I was trying to do but I was in a computer class in high school, doing something with BASIC. What I did was basically is, the same window opens infinitely until RAM is full, then Windows crashes into BSOD. It was a fun view. Tried it on another PC in the class to be sure too, transferred it via a diskette.
The only major accident I’ve had is when I was 17 and worked over the summer for a major auto manufacturer. I was tasked with writing a piece of software to make inputs easier to a bash script some nerd made on his own free time that they now pretty much relied upon since it saved them several hours each time they run the script.
To try and make a long story short(er). I had access to a server, that I was told was a temporary server, and there was nothing important on it, I had access to create and run files. I did not have access to monitor, kill, or terminate any processes. I made a silly mistake and created an infinite loop, but it wasn’t your average infinite loop. It was the worst kind of infinite loop imaginable. Took a few executions to diagnose where exactly I had fucked up. I fixed the problem, no more infinite loop. And it actually worked. But these processes still sat there in the background. And they grew, and grew, and then another server started helping out to try and save his server friend. And when that wasn’t enough, a third server tried to help, and so on.
I noticed that things were starting to run a bit slowly when browsing the server for files, others complained as well that the servers were slow today. And then I get a phone call from the head of IT, asking me if I need all of these thousands of processes running to which I shamefully replied “no…”. But it was too late, and so much had stopped responding that they could not kill it. They pretty much emergency shut down two thirds of all their servers in house in order to save the rest. I had pretty much created rabies but for servers. Anyone infected had a death sentence. It was only a matter of time. Pretty much anything, anyone had worked on that day was gone.
I had a coffee with the head of IT later, He didn’t blame me and was really nice about it. The data on the server I was given access to was not important, that’s why they let me use it. But they had not considered that the server load when above the threshold is then shared to other servers, and that they should have isolated the load from that server as well so it doesn’t spill over to another. It was a genuine lesson learned for everyone.