As I'm sure, we have all given someone else an OS for their calculator. The reasons for their OS going awol can be caused by programs malfunctioning or somthing else.
The strangest one I have seen was from a friend, who came to me with their calculator (TI-84 Plus with a 2.22 OS I think) who had loaned it to a friend. It was just for math. The story I got was that the friend had done the Square Root of 55 Squared. And somehow, it dumped the OS.

If you have any idea why, or other strange things that have dumped an OS, please tell.

(√55)²? seems to me that the OS was dumped because of extreme insult. Humor aside, I did dump a friend's OS by accident once (don't worry, I got it back) by giving him MegamanZ.
EDIT: the square root thing is there because I made this post with my 3DS so it isn't exactly MathPrint looking.