Hardware malfunction
While I was preparing the previous entry, my laptop had a nasty hardware malfunction: it was turning off abruptly while I was typing, often with some flashy visual effects (colorful "white noise" combined with bands running over the screen). My first guess was that the power plughole got loose and some contact was touching the display cable contacts (as I knew before, it is running near the power plughole) and a short circuit occurred, but it proved wrong: even if I tried working with the laptop running on battery, it was still occasionally turning off. It was even more strange that the slightest key-press could cause the laptop to shut down.
Finally it drove me crazy and I decided to wield a screwdriver. I took off the back panel and after some investigation found nothing: the power plughole was isolated safely and couldn't cause any problems. The battery socket was as secure as the power plughole. Just before I took the back panel to replace it, I caught sight of the offender: it was a screw which had been securing the motherboard inside the frame before it came unscrewed and it was then rolling back and forth across the motherboard occasionally causing the short circuits.
After I reassembled the laptop, I recalled that the previous malfunction of this laptop was caused by the screw as well: my DVD CD-RW was misteriously failing to work a couple of minutes after I turned on the laptop. The reason was the screw in the back panel to which the drive was hooked and the screw got unscrewed and gone.