Well, I'm not gonna tell you about one embarrassing hack, as I honestly can't remember about one from the top of my head, but I'll tell something someone did to me:
While at school, working on our final project, me and my colleagues had to build a simulated enterprise network. In between installing OS's and services, and configuring them, I often found myself bored. So one time, I noticed that a colleague was using one of the project computers (which was running debian) to use facebook. Since we had ssh access to every computer, I ssh'd to his computer, ps aux | grep empathy and kill -9 <empathy PID> (yes, it's not a real hack since I already had access, but that's not the point, keep reading). This happened about three times before I busted laughing at his "WTF is happening" expression, which gave me away.
Anyway, he seemed like he didn't mind much, and after a while I went outside for a smoke. When I got back and sat in front of my laptop, I noticed the USB keyboard and mouse I was using weren't working. Since it was a laptop, I just used the touchpad and built-in keyboard to find out what happened. I knew he obviously did something, so I started acting all like "meh, I'll solve the issue in no time". Well...
While using the touchpad, I noticed he had taken a printscreen of my desktop and set it as the wallpaper, hid the taskbar on top,... Oldest play on the book, easy to figure out. Little I knew he had a trick up his sleeve...
Next, I checked the base of the mouse for duct tape, checked if the cables were cut, checked the device manager for uninstalled drivers, re-installed keyboard/mouse drivers, checked USB drivers, re-installed USB drivers, switched USB ports, booted Linux instead of windows... I spent 30 minutes scratching my head, but nothing fucking worked.
When I finally gave up, he looked at me smiling and said: "Look at the USB connector...". Fuck me! The bastard had cut two small pieces of duct tape and placed them on top of the USB contacts of both the keyboards connector and the mouse connector. Shit, I never would have thought to look there! I felt so embarrassed...
Well, at least I learned a new trick