Dumbest Computer Failure You've Had

So I just upgraded my desktop yesterday. Installed a new Z77 motherboard, i7-3770, and 16GB of RAM. I don't build computers very often, so I was pretty meticulous during the installation, double-checked everything, grounded myself constantly, etc...

Flip on the switch, and there's a weird grinding sound, then two seconds later an incredibly loud pop and a flash of light. Shit.

Turns out that my OEM Intel heatsink fan had had the power cable wound too tightly around its clips, so it was interfering ever-so-slightly with the free rotation of the fan blades. I guess the high-speed, low-torque motor tried to draw way too much starting current, overloaded the PSU and then KAPOW!

Fortunately it was only a $75 mistake for a new PSU and not a $500 one, but goddamn was that a piss-off.

So what's the dumbest computer failure you've had? Could be hardware, software, something you did, or something that's happened to you.