Mona Lisa Secret
From Screamer Wiki
Mona Lisa Secret is a video posted to YouTube in June 2006, making it one of the earliest YouTube screamers. It was created by user Kill3r24akaAdam.
Video
Simply, this video claims that in the famous Mona Lisa painting, there is someone being murdered off in the background, and invites the viewer to find them. Instructions are given to look closely at the screen.
Then, the painting appears, with the screen constantly zooming into it. At twenty seconds in, a girl in a zombie mask appears with a loud growl. The credits then roll, with the user claiming he didn't come up with the idea,
Link
WARNING: The following video contains a screamer.
youtube.com/watch?v=7cjRmuljAKc