Palpatine's Resurrection in the Rise of Skywalker Broke a Major Sith Rule
Emperor Palpatine, aka Sith Lord Darth Sidious, is infamous for returning from the dead both in the old Star Wars Expanded Universe, and in Episode IX of the new Disney canon.
Both cases involved preparing in advance a secret hideout, which doubled as a superweapon factory, and creating a clone body for himself to inhabit. In both continuities Palpatine had prepared well to ensure that he, of all people, won't die when he is killed.
But what if we told you that in doing so he broke a major Sith rule? At least according to the Star Wars Tales #9 comic book, which was written by Ron Marz and illustrated by Rick Leonardi, back in 2001.
That issue of Star Wars Tales covers a battle between Darth Vader and Darth Maul, who returned to challenge Vader for the place of Palpatine's apprentice. During the battle, Darth Maul takes out a bridge that causes both him and Darth Vader to drop.
When Vader manages to land gracefully, Darth Maul cites a teaching from Palpatine that Sith are never supposed to break a fall because if they do so that means they are prepared to fall in the first place. "Sith do not fall," he concludes.
Vader in return compliments Darth Maul, saying that Palpatine taught him well and confirming that this rule about falling is indeed a part of the Sith doctrine. Yet Palpatine himself was clearly prepared to fall (though that did not save him from being killed by a very literal fall, of course).
Before pondering it, you have to consider two facts about this interesting little tidbit of lore. First, it is a part of the old Star Wars Expanded Universe, now relegated to the "Legends" label and effectively treated as non-canon. Second, even in the Expanded Universe, the Star Wars Tales comics were only quasi-canonical.
Disregarding that, yes, in the old continuity there was a Sith code, and there were multiple suggestions that Darth Sidious might be called a heretic by Sith standards – he did not follow the usual rules of the Sith, which he taught to others.
However, you also could say that the true essence of the Sith is dedication to power and victory through any available methods, and Darth Sidious was so supremely powerful in the Dark Side, because he grasped that essence, while his apprentices still allowed themselves to be shackled down by rules of others. An interesting conundrum to think about, isn't it?