Those are fun to play with. I knew a guy who had a computer science degree and came back from the US with an AOL CD and screamed at Freeserve (yeah you can tell how old this story is when Freeserve is mentioned....), he insisted AOL was better and he was going to install it. So he was bored waiting for the installer to finish and had a PSU with a voltage switch on the back of the case of a high end academic machine. He started flipping the switch to pass the time. I got the hell outta there since I knew what it'd do. Took a few steps outside the dor, boom. Swiftly followed by various components giving up on being components.components..
Also, www.rinkworks.com/stupid has some awesome stories, but....that one about hitting all the computers in the lab was hilariously awesome.
I managed to somehow, amusingly, kill an entire supermarket by simply scanning an item. Sainsburys in the UK had, for a time, hand held barcode scanners. You'd go scan your item, it'd add it to the memory. You'd connect it tot he checkout or hand it to the cashier. So, was scanning items when I didn't ear the little beep that confirmed something'd been scanned. THinking it was an error I tried again. Little did I know I'd broken the entire supermarket system. I'd scanned something that'd been mislabeled in the system and it'd apparently gone and broken the table. they were stored in. Now, the store this was in hadhad some stuffed toy Aussie drop bears. Bear in mind...this was SQL databases. So by the time I'd found the drop bears they'd fixed the broken table. However...scanned the drop bear in. Boom. Everything just fel completely over. Turns out I'd told the database to drop everything past the drop bears...and since it was in the D range....well......everything but the air conditioning in the store gave out. Dors were locked, no power, no electronic stuf, no intercom....it made for a rather hilariously awesome moment. Apparently even big supermarket chains aren't immune to database screwups after all....