NightMare

NightMare was a scareware application created by Patrick Evans in 1990.

In the Amiga Disk Operating System (DOS), when the person executes the code: "Nightmare", it will run in the background, in every 5 minute, at which point the screen cuts into a illustration of a skull, seen from its right side, with its mouth open, teeth covered in blood and a bullet hole on the back with blood leaking from it. This is accompanied by an echoing shriek and happens for a four-fifth of a second.

Links
NOTE: The following video contains a screamer!
 * youtube.com/watch?v=ubFGj4lQSyM