This is an absolutely hilarious prank.
Flash 9 comes with a fullscreen mode. It’s used in the YouTube player, BBC iPlayer and so on. When fullscreen mode is opened, Flash displays a semi-transparent message telling you full screen has been activated – the reason for this is so somebody couldn’t spoof a user interface using Flash to steal your personal information.
This prank distracts you from that message and proceeds to scare you using full screen mode. Press Escape to exit.
The only way this could be improved would be with a message saying “Press any key to restart your computer”. That would scare anybody out of pressing Escape. Although it would certainly cross the line of being very evil!