Title: The Shining (1980)

All work and no play makes Jack a dull boy

Actors: Jack Nicholson, Lia Beldam (young woman), Billie Gibson (old woman) 

Scene: Jack Torrance goes to room 237 to investigate this mysterious woman Wendy accused of physically assaulting their son, Danny. Instead of getting upset at this lady upon finding her in a bath, he is rather smitten with her. As a result, we are taken aback and deem Jack Torrance a very unfit husband, and an extremely bad parent in one take. Nonetheless, it's the reflection that Jack sees in the mirror that beckons the ultimate WTF moment.

Why: One of the most WTF scenes ever on all accounts!