During the end credits, as the actors image and name appears, there is a lightning flash on the actor's face. When this occurs, an image of a "skull" appears superimposed over the face with the flash and then fades.
The credits show images of the actors. When each image appears the skull shows first then the rest of the actors image fades in. When Arnold Schwarzenegger's image appears the top right hand side of his face is last to disappear and there is a light in his eye. Referencing the poster for The Terminator where you can see the metal skull and glowing red eye.