I think the movie line is pretty famous but can't quite remember.
The speaker talks about some kind of an effect and he drops a ball down the slope and says something inevitable happens no matter what, meaning that the ball will fall to the ground anyway.
And it does.
When the listener tells him that he could have prevented it by catching the ball before it touches the ground the speaker says, "But I didn't."
Well, the actual movie line might be very different, but this is what I can remember.
UPDATE: (I'm adding a few more details as others requested)
The movie is in English and I believe it is an American film.
I can't remember the actors. The movie scene where two people are talking is a dark room (and there probably were a few more) and I think a projector was running.
I didn't watch the whole movie but that particular scene was cut and played in a TV show or another movie, I can't remember. The time I watched this was around 2009-2010, but I felt that this was somewhat popular a movie scene that it was quoted and played - the effect being explained popular like "butterfly effect" - and maybe produced sometime earlier. (not sure again when)