This was a complete revelation when I originally got it - you just plugged it into a computer and the computer recognised it and you could play tunes, without a MIDI convert or a mains adaptor or anything. Until the day I trod on the cable and broke some kind of connection inside, which meant that the USB bit didn't work any more so I did need to find the MIDI converter and the mains adaptor and a bunch of cables. Hum.

Then I tripped over it and broke one of the keys so that it stuck out of the keyboard at a jaunty angle.

I've had a replacement for a while now, but have been trying to convince myself that I would never plug it in again (It does sort of work, but only under very particular circumstances).

Another category: things that are broken or useless but were originally so expensive you don't want to throw them away even if they'll never be used again.