Luke Skywalker may have struggled with the dark side in The Empire Strikes Back, but it was actually Yoda who was in danger of falling. As unbelievable as that may sound, there is quite a bit of evidence to suggest that Yoda was indeed falling to the dark side, fulfilling the very warning he gave Luke. Viewing Yoda's character through this lens recontextualizes all of his scenes with Luke and adds more depth to the overarching story of the original Star Wars trilogy.
Yoda falling to the dark side would also tie back to the Star Wars prequel trilogy and the failures of the Jedi Order. The Jedi's vision was clouded by the dark side of the Force, allowing them to become embroiled in a war that blinded them until the Republic was gone and the Order decimated. This adds credibility to Yoda being the one who was falling to the dark side and would make George Lucas' saga about the Jedi Order's redemption, not just Anakin Skywalker's. However, The Empire Strikes Back contained plenty of evidence on its own, hinting that Yoda, not Luke, was falling to the dark side on Dagobah.
Jedi Don't Deal In Absolutes – But Yoda Did 
"Only a Sith deals in absolutes." These were Obi-Wan Kenobi's words to Anakin on Mustafar, but Yoda showed that even a Jedi Master could fall into this trap. Yoda's dark side warning to Luke was right, explaining how negative emotions could easily lead a Jedi down the wrong path. Even so, his words to Luke sounded awfully like an absolute: "If once you start down the dark path, forever will it dominate your destiny, consume you it will, as it did Obi-Wan's apprentice." Not only was this the beginning of the impasse between Luke and his masters over Darth Vader's redemption, but it also exposed Yoda's own fault.
Yoda dealing in absolutes is very similar to what the Jedi claim is a Sith way of thinking, which suggests that Yoda himself was struggling with the dark side. Obi-Wan's line also demonstrated that Yoda and the Jedi struggled with this during the prequel trilogy, as his line to Anakin was itself an absolute. Even Yoda's most famous line, "do, or do not, there is no try," is an absolute. While it was meant to encourage commitment, it could've led Luke down the wrong path later in his journey. Luke was constantly given two choices but found a better way, demonstrating the danger of taking Yoda's advice too literally.
Yoda Was Consumed By Fear… Of The Dark Side 
Yoda also warned Anakin that "fear is the path to the dark side," yet Yoda himself was clearly afraid. Yoda refused to train Luke because he was afraid that he'd turn out like his father. He was afraid of the dark side when he quickly dismissed Luke's question about whether it was stronger, almost as if trying to convince himself. Finally, he was afraid of what would happen to Luke if he left to save his friends and of Vader and the Emperor not being stopped. Yoda's fear was completely understandable, but as the Jedi Master himself said, "fear leads to anger, anger leads to hate, hate… leads to suffering."
Star Wars Proved Yoda Wrong When Anakin Skywalker Was Redeemed 
What truly suggests that Yoda was falling to the dark side is that Star Wars proved he was wrong about Anakin. Yoda believed that it was impossible for someone to return from the dark side once they'd started down the path, and both he and Obi-Wan believed that Luke needed to kill Vader. However, Luke listening to his teachers would have been the very thing that sealed his own dark fate, as killing Vader out of anger was exactly what the Emperor wanted. By rejecting what everyone else thought was best, Luke chose love and non-violence, proving himself a true Jedi and inspiring his father to return to the light.
