[Jonty30]

In the original run, there were extra frames showing a skull added between the movie frames. It wasn’t there long enough to be seen by most people, but long enough that it still registered in your mind, helping to create a sense of fear.

http://www.eeggs.com/items/3204.html

[CaptainK]

They weren’t that subliminal because I remember seeing that white face picture in the frame to the left.