What was a moment where a not-so-serious movie took a quick turn for the very serious?

I'm thinking of the face-crushing scene in Pan's Labyrinth. Not that it was the most happy and frivolous movie before that scene, but the intensity and the stakes jumped massively the moment that dude got his face smashed in with a bottle. What are some other examples that made you sit up and say 'holy shit' where a movie suddenly raised the stakes in a moment?