Crawford on the Beach [spoilers]

At the end, when Joan is walking on the beach in a glamorous evening gown, she passes a man and his dog. She walks past them, looking straight ahead. Her suffering becomes too much so she decides to drown herself, as the Wagner violin piece reaches a crescendo. Then a little way down the beach, the music softens as the man turns to toss a stick to his dog and realizes she's disappeared.

Here is a modest-looking man enjoying a peaceful day by the sea, playing with his dog, having a wonderful time. Meanwhile Joan, dressed for an elegant party, is about to end her life. I thought these contrasting images, combined with the music, are some of the most haunting I've seen on film.