Back to the Future
Marty McFly went back in time and met his parents when they were in high school... So, why don't they recognize him as that "weird" kid from that one night? It would only make sense that they feel some sort of deja-vu while looking into the boy's eyes. Some fans speculate that they do recognize him, but it's a stretch.
The Wizard of Oz
The Wicked Witch dies after a pale of water is thrown onto her... It seems a little crazy that it was that easy to kill one of the most powerful evil villains in the land of Oz. Did it ever rain in Oz? Was she always at risk of being totally smoked?