I got the UN500 error last night after deciding to experiment with the olympic interactive coverage on BBC2. Had to unplug the box to reboot it. I won't be doing that again; the plug is behind a childproof locked cover, I can't be bothered crossing the room and unlocking it every time my STB grinds to a halt. You may think this shows lack of patience on my part, but in the three years or so I've had NTL digital, the STB has crashed so many times, I just can't be bothered fighting with it any more.
I have lived in hope that the interactive services would one day work consistently and well. I'm moving house out of the NTL network in a couple of months so it looks like I will never see the day.