2. Star Wars: A New Hope
This one's huge, and anybody that has seen any sort of Star Wars parody(Family Guy, I'm thinking) will know about it. It is, of course, the incredulous and illogical decision of the Star Destroyer Commander to NOT blow up the escape pod containing both C-3PO and R2D2. Let's be fair, everybody loves to blow things up, especially when you are controlling a high powered laser, and when an escape pod jets off a ship your forces are invading, it really does make sense to give it a good old blast. Seriously, it would have been so easy. For the films sake, George Lucas could have left out the scene in question and just had the escape pod sneak off without an explanation, but he didn't, trying to rationalise it by having the Commander say there were no signs of life on board. IT'S A GALAXY WITH A HIGH POPULATION OF DROIDS. Without that, the droids don't meet Luke, Luke doesn't meet Obi-Wan, and the saga doesn't happen.