I was messing around the other day on my calculator and I discovered an interesting bit of code:
With an answer value of 12, 13, 14, or 15, the screen's orientation is flipped to normal, backwards, upside down and backwards, and upside down (respectively). With 64-128, the screen is vertically shifted, and with 220-256, the contrast is adjusted. Just something interesting.
This only works on pre-MP calculator's, as I had tried to prank a friend running 2.53 and the flipping didn't work. (The shifting, however, did work.)