Title: Scanners (1981)

Stopped me dead in my thoughts for at least ten bloody seconds.

Scene: 

Why: David Cronenberg's Scanners is one of those movies that has absolutely embodied the WTF concept in virtually every segment of that film. This scene above is one of the most revered WTF moments which provoked any other horror movie from the 80's to meet or exceed that level of gruesomeness or go home.