When Darth Vader and Darth Maul face off in battle during a Star Wars Legends, they reveal that Emperor Palpatine isn't as dedicated to Sith doctrine as he would have them believe.
Emperor Palpatine, known by the Sith name Darth Sidious, has had several apprentices throughout the Star Wars series. Besides Count Dooku, both Darth Vader and Darth Maul were tremendously strong. Vader, of course, was the doomed Jedi Anakin Skywalker while Maul was struck down by Obi-Wan Kenobi on Naboo. While the Death Star was being built, Vader was lured to a planet under false pretenses that a Rebel cell had stolen Death Star plans and was hiding out there. In truth, it was a trap so that Darth Maul could face Vader for the honor of being Palpatine's true apprentice.
Taking place in the Legends story Star Wars Tales #9, which was written by Ron Marz and illustrated by Rick Leonardi, the fight is fairly evenly matched. During the battle, Darth Maul takes out a bridge that causes both him and Darth Vader to drop. Darth Maul notes that Vader lands gracefully. Darth Maul then cites a teaching from Palpatine that Sith are never supposed to break a fall because if they do so then they are prepared to fall in the first place. "Sith do not fall," he concludes. Vader then compliments Darth Maul that Palpatine taught him well, confirming that this fact about falling is indeed part of Sith doctrine.
The strange thing about this rule though is that Palpatine betrays it himself. He is quite literally killed by a fall in Return of the Jedi when Darth Vader throws him down a reactor shaft in the second Death Star. And not only is a fall the method of his demise, he is also very much prepared for it, so much so that he clones his body in order to eventually rise again years later, as seen in The Rise of Skywalker.
This feeds into the theory that Emperor Palpatine was actually a Sith heretic at heart. In the early days of the Empire he actually tried to replace Vader using technology developed by a scientist named Cylo. At the time, Darth Vader declared this Sith heresy. Then again, since Sith cherish survival and conquest at all costs, perhaps there is no real Sith doctrine. All that matters is winning and destroying the enemy no matter what. In that case, it's something that Sith lords like Darth Maul and Darth Vader have in common as they fight for Emperor Palpatine's approval.