Try this on a 15C: Hold down "-" while hitting "ON" (this will clear all of your memory). With Pr Error displayed, turn the calc off. Turn it on again, press "4" and turn it off again. Now hold down the square root key (upper left) while turning it on, and turn it back off, 5 times. Display should read "b 0 0". Hit "+". Some program runs, and the display reads "A 0 0". Now press and hold "+" or "-" for about 5 seconds. Display reads "null" like a 41 would. I don't know what good any of this is, but it's weird.
HP-15C key sequence
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