The worst that I had is... I captured a virus once and kept it there just in case so after a few weeks i forgot what it was and opened it, 5 seconds later... SYSTEM32 is GONE....
My friend threw a snowball at his brother but his brother opend the door at the same time and the snowball missed and nailed my macbooks screen dead center, then it fell onto the keyboard. Not to worry though, I turned it upside down (yes it was still on but it has an SMS sudden motion sensor) and put some paper towels on it. Still works fine. Love the fact that the screen is covered by glass.
One day sitting enjoying some good YouTube fun videos on my Toshiba Laptop, drinking a nice cool Coca Cola light drink........ And yes. In an attempt to reach for some kind of snack, I knocked my drink right onto the laptop.
I removed power and the battery ASAP, and crossed everything that can be crossed and hoped for the best. 2 days of drying. And everything worked except the monitor. And I actually think it happened when I turned the laptop upside down to get the battery out.
Anyhow ... luckily all my data was still there. I normally remember to back my stuff up, but when this happened I had used my external drive for something else. DOOOOH!!
I built a nice system in 2007 ($150 CPU, $100 mobo, $80 in RAM, $90 graphics) and didn't have the money for a hard drive at the time. Nobody was kind enough to spare a hard drive, so I took the nicest one I could find and used it for 3 months. It was a 4GB drive from a '99 Dell, and I killed it with a screwdriver.
I've got too many to count, but my most recent one is accidentally snapping the spacebar on my Apple keyboard in half. Now I'm stuck using the page up key as a spacebar.
Of course there are better ones. For example, when I was about 3 years old (we had just gotten a Windows 95 PC) I took a magnet and was having fun with the CRT screen seeing all the colors go crazy. So I put the magnet on the side of it and like the CRT sorta like exploded in my face and I was banned from the computer (and magnets) for a year.