This most often happens when you are running an Assembly program. It is generally accepted that Assembly made programs should have a "Z" in front of them, but some don't. You can tell if the program is an Assembly program or not in the listing of all items on the calculator. It will have an "ASM" next to the program instead of "PRGM," if it is an Assembly program. You can access the listing by presing and [MEM]. Scroll down to 2:Mem Mgmt/Del... and then choose 1:All. To prevent this from happening, it is a good idea not to run Assembly programs that you have any doubt about. Also, always have your important files backed up in the archive or you computer.