#1.Star Wars: The Empire Strikes Back
We had to make this number one, not because of the size of the plot hole, but because it's friggin' Star Wars. That's right nerds, the indisputably best one of the series has a pretty gaping hole of its own.
You know the plot. Don't play that game.
So there's the famous sequence where Luke gets trained by Yoda on Yoda's shithole of a planet. To break up the sequence, the film cuts to the Millennium Falcon getting chased by the Empire to Lando's cloud city. When they arrive, they get captured, at which point Luke has finished his training.
Well, that doesn't work. Were they chased for months? Or was Luke trained in an afternoon? Either we were spared some extended scenes on board the Millennium Falcon featuring starvation and debates about when they'd have to eat Chewbacca, or becoming a Jedi is easier than getting a cub scout merit badge.
The latter explanation seems more plausible, as it just reveals Luke to be an even whinier bitch than he seemed. Talk about ungrateful, he's getting taught God-like abilities in about six hours, and he complains through literally every single one of them. It also means Yoda's insistence that Jedis start their training as young children isn't because the training's such a long arduous process, but because he's amused by the idea of children knowing how to choke each other with their minds.
Now it's true that when Luke tries to leave, Yoda insists the training isn't over. But when Luke returns to Planet Shithole in Return of the Jedi to finish it, Yoda waves him off and tells him there's nothing else to learn.
Then it turns out the final test Luke has to pass to become a Jedi is to defeat Darth Vader, the most powerful Jedi in the universe which kind of seems like a huge leap in difficulty after his one-day training session. That'd be like if the final stage of your driving test was to win the Indy 500.
So to answer the question, at what point did George Lucas stop paying attention? It looks like it was part way through the second movie.
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