The Sound of Music, surprisingly. The first like 90% of the movie is them singing and frolicking through fields, then it ends with them being chased by nazis. It's such a radical shift in tone.
There are a few hints at the Nazis early on and throughout, but yeah. The last 15-20 minutes are car chases, tense hiding scene, and a standoff. Nothing like the movie up to that point.
I assume it's probably meant to be like how it happened in people's actual lives. Rumors of Nazis, things happening, and then suddenly they are there and your life has been completely upended.