In 1990s we were writing a virus at school which applied this algorithm to all characters on the DOS screen after some period of inactivity. Destroying Norton C

In 1990s we were writing a virus at school which applied this algorithm to all characters on the DOS screen after some period of inactivity. Destroying Norton Commander on a computer used by a teacher or sysadmin was quite fun (the virus had to be manually installed, did not survive reboot and screen was restored on keypress event, so it was quite harmless). Nobody punished us for this kind of jokes.