Mashable.com posted a video of movie moments that were improvised. The most famous is probably "The Empire Strikes Back", where Princess Leia says, "I love you," and Han Solo replies, "I know."
Others include:
The Joker's slow clap in "The Dark Knight",
Leonardo DiCaprio accidentally cutting his hand open in "Django Unchained",
Martin Sheen accidentally cutting HIS hand by punching a mirror in "Apocalypse Now" . . .
And the awkward hug between Voldemort and Draco Malfoy in "Harry Potter and the Deathly Hallows - Part 2".