Watching Infernal Affairs
, I really jumped at the moment where the police boss comes falling off a skyscraper and onto the roof of a car (and making the car horn blear). I just called out "HEY!" (in English, meaning "wtf"), mouth open - and then a few moments later, giggled at my own reaction. Normally I'm a well mannered film goer who doesn't make any serious noise in the theatre.
The scene really takes you unprepared when you watch the film for the first time (it's not on Youtube but the immediate aftermath is here: Goodbye Policeman (Zai Jian Jing Cha)
and you can see his broken body; brilliant film). It's set just after a key confrontation on the roof of the high-rise, a meeting between two cops working undercover but in different ways, and the police chief is not directly involved up there, so when that scene ends without anyone dying and the two men pull off, you breathe out, mentally. Then out of nowhere the big man falls on the car! It's such an insanely unexpected twist.