I just saw this video and it's amazing, you can download the source code and the instruction to build the robot if you're an NXT fan. (http://tiltedtwister.com/)
I hope you enjoi it.
Did you never "solve" a Rubik's cube by peeling the stickers off? I guess he couldn't be bothered sticking them all back on. Perhaps the glue all wore out and so he gave up on that an made his NXT solve it instead?
I can't think what the colour is which was missing, but perhaps there wasn't enough contrast between all the colours for the sensor? Yes. That seems more likely!
Whatever: it's cool.