I had a client who refused to pay for his PC after our shop had done repairs, so just before he came to pick it up, I filled the case with loose stale popcorn. I'm sure he could smell it when it heated up.

Another time I had an endless looping batchfile running silently, scattering endless blank text documents across the desktop faster than you could delete them.

 

Once when I was going on a 3 day trip, I set up my pc to where it would hog up all of the internet bandwidth so that my sister could not watch anything online since the internet was too slow.

 

I smashed my bricked Wii. Right when it bricked I went outside took out a bat and now it's just a bag of garbage

 

I have the Wi-Fi disable internet access from 10pm until 9am, when our teenager should be sleeping until they go to school. Its so satisfying.

 

Maybe I should do that to my parents and sister?
Would be quite fun.

 

1) I set my router to throttle all the bandwidth for my brother's Xbox One, as he was using all the bandwidth to play Destiny, killing it for the rest of us, not to mention using more than 50% of the bandwidth we get a month. We live out in the middle of nowhere so we pay a lot just to get good high speed internet.
2) Inversely, I've configured my router to give my devices unlimited/priority bandwidth for the entire network. It's good to be in power.
3) Nothing else yet. But I'm always plotting world domination. Stay tuned.

 

hehe I could do that to any machines except the one I chose

I could annoy my GF a bit.

 

I uh... well, maybe this is TOO Evil to post, buuut.... my friend of over 10 years and I found out his brother was sleeping with his girl. he asked us to fix his laptop cuz he had too many viruses... so I installed Windows ME on it, made a batch file to run automatically and download about 200 different Trojans, and about 50 or so worms/spyware/malware, etc. I was then able to access his financial accounts and donate all his money to about a dozen different charities. I also put my first Wii into a microwave. fun. after the wicrowaving, I took it into the driveway, poured gasoline, gunpowder, and windex on it, then placed some leftover artillery shells from the 4th of July all around it and threw a match on it. instant fun. I also once bought 5 copies of Superman 64, and didn't destroy them. Instead, I made some Japanese labels of Majora's Mask and switched the labels, and sold 3 of them to some buddies overseas, sold 1 to a collector who never plays his games, and kept the final one to remember the fun I just had... and then baked it into my little sister's birthday cake. good times.

 

1. At school I replaced my teacher's PC's screen saver with hardcore porn. I of course password blocked it. Eventually he was broadcasting to the big screen in the class, he left his PC alone and the screen saver broad casted on the big screen. Many laughs were had.

2. One time when I was at the library with my friend, i turned the volume up on all the PCs, not realizing what my friend was planning to do. My friend went onto the library PC, and went to some hentai site, and suddenly "OH! OH! KIMOCHI! OH!" being blasted through out the library. My friend tried to close it, but this was like 2004, so the PC was shit, and the Internet Explorer browser of course locked up, while playing the sound still. The lady at the library was so mad, and threw him out, and said he was banned from there. Good times!

 

I don't have as much of those... :-(

I think these are about all of them:
-one time at work someone was making fun of me behind my back as I was working on his computer. I didn't say anything of it, but afterwards I got into his computer by \\<computername>\c$. he got a few games installed that...somehow ended up missing a few files from them.
-in another job there was this one guy whom I couldn't stand and who should have been fired on the spot for even half the stuff he did (but there he somehow ended up running a facility). For some strange reason, he not only had admin rights on his own pc (this wasn't a relaxed company like the one described earlier) but actually had WBFS manager on his system. I knew that if I confronted him with this he would complain about me behind my back, so I just removed one essential file from it (a .dll file, iirc). A couple months later, I did the very same thing.

And there was this one thing that may have been evil, but was something I had to do as administrator. You see, that second job also had a very weird labyrinth of folders and subfolders to which divisions or users had access to through some very obscure rights nobody even pretended to understand. Nonetheless, we as a company were charged on a monthly basis for the stuff that was on it. Not especially high, but enough to use spacesniffer to figure out which were the highest ones. And sure enough, somewhere in those folders was one that was so huge it took over half of the...perhaps thousand different folders. It was filled with ripped movies. Some made years before, some as recent as a few weeks before.
As this was a retarded company, there was no way I could file a complaint (it would've backfired into me not having properly maintained a structure I only inherited from someone who had to do file administration as a side job as well). And I really wouldn't want to piss off people I had to work with. So I put the whole folder in yet another subfolder, and put a note in the folder itself saying I understood why they wanted to do it, that I wasn't the police, but whether they would please not use the network drive. I'm not sure whether I mentioned it anymore, but the price our company paid just for stocking (and probably daily backups as well) for that folder was well over that of a monthly DVD.
As I expected, nobody replied or wrote anything back. So a couple weeks later, I just removed the whole folder.