Had a screw floating around somewhere in my case (you can guess where this is going!) It was fine for a long while, until one day I decided to move my rig to a standing position. I Heard that darn screw rattle around and get stuck somewhere. Somewhere behind the mobo, and I didn't feel like unscrewing everything to get at it.
I figured, "It'll be OK." Hooked everything back up, and hit the power button. POW!!! Something on that board went off like a firecracker! I saw a jet of flame shoot out from my mobo, like a little solar flare. Then, suddenly, all was deathly quiet, as smoke and the acrid smell of burnt electronics filled the room. Uhh... it was time for a new mobo anyway...
Sure enough, upon unscrewing that board, that screw fell to the bottom with a pleasant little "clink." Almost like it was trying to apologize for frying my board.