The scene where Private Pyle in Full Metal Jacket goes off the rails. The man's acting was phenomenal.
I thought the movie would've been a comedy at the beginning, especially with the Drill Sergeant's over-the-top insults, but that scene just left me stunned. It really subverts your expectations; Pyle is the kind of character that would normally prove himself and become a hero in other movies. In a way he kind of does become what they wanted him to be, but at the cost of his own mental health and sanity.
This is one of the reasons why I love this scene. Pyle shows that some people just aren't cut-out for certain things. They can't help it, and sure, they might learn over time, but everyone has a breaking point, and this scene shows that they had went passed Pyle's long ago.