Hacked TIs with "fake memory clear" and other clever tricks were/are pretty common too.
But the most impressive I've seen were some calculators I saw in Shenzhen a few years ago which looked like simple solar-powered 4-function ones, but were actually programmable and had several tens of KB of memory. Trying to use that functionality with a 8-digit 7-segment display, however, was quite challenging.
If you ever stumble on old or further information about these 4-function calculators, I would love to learn more. That sounds amazing.
Did the solar panel actually power it, or was there a hidden battery?
heh, i remember writing a fake "memory clear" program on my TI back in 1994/1995 (i don't remember the exact model).