2 Luke And Leia - Return Of The Jedi
Another big one: when Luke finally finds out from his dead Jedi mentors that he has a sister, he immediately senses that she is Leia. Though as she's pretty much the only female in the original trilogy, Luke doesn't exactly have to use his Jedi mind tricks to make the connection. This sort of information would have been helpful to everyone in the movie universe: there would have been less mooning around with crushes, and sister-kissing, and more concentrating on saving the galaxy, and Han would probably have loved to know. In our universe, however, the surprise changed everything in the rest of Return of the Jedi. Yes, we all probably went back to the moment when Leia kissed Luke and how he spent the entire first film pining for her - especially, as Leia announces she probably always knew - and it did make the odd little love triangle a little bit weirder, but it didn't change the impact. Unless you had already seen Revenge of the Sith, in which case you'd already spent the last two and a half films cringing every time that the twins got a little bit too cozy with one another and wondering when Vader would hurry up and find out that Luke wasn't his only surprise child, having somehow never found out about the twins (bad parenting, not attending the scans.)