As it has been noted by many, including Lucas himself, the concept of "what really happens in Star Wars" is actually two things: the movies and those works that are direct by-products of the movies, and everything else: books, comics, television shows, games and any other products LFL has given at least tacit approval, at one point or another.

Accordingly, the folks that Lucas has left in charge of determining "what really happens in Star Wars", otherwise known as continuity, have a distinct division among the sources between what Lucas has told us, and what anyone else has told us. The former is "canon" or "movie canon" or "G-canon" depending on the source, while the latter is "official" or "quasi-canon" or "expanded universe canon" or "C-canon". Of course, "canon" always takes precedence over "official". Furthermore, these folks who are in charge of continuity have made it clear that "canon" does not just include the direct artistic work of George Lucas, but also the expressed wishes of Lucas regarding the direction he wants Star Wars to take.

The sharpest conflict where this comes into play is with any appearance of the Sith that occurs after the events of Return of the Jedi.

George Lucas has made his intention clear, that when Anakin Skywalker becomes redeemed through the actions of his son, Luke, and subsequently kills Emperor Palpatine, the Sith have been destroyed, apparently forever. However, in the Expanded Universe, the Sith have been destroyed many times before, and in each case, they were reformed at a later time. Sometimes it happens quickly, and other times it takes centuries. The same can be said for the Jedi - they have been destroyed many times, and each time they have returned.

However, the intention of George Lucas on this topic has already been "officially" undone. It was even part of the first LFL-directed effort towards continuity. While the original Star Wars trilogy was being produced, books, comics, cartoons and games each told their own little (or not so little) stories within the Star Wars galaxy. While they did not especially tread on each others' toes, they also did not coordinate their efforts. Following this, West End Games published a Star Wars role-playing game, whose writers attempted to find a continuity between the various sources that had come before. During the game's publishing run, there was a period when no other Star Wars products were being produced - there were role-playing supplements about the Star Wars stories, but no new stories. When new publishing projects were being planned in 1988, Lucas Licensing wished to place these projects within the sense of continuity that existed in the role-playing game. So the Thrawn Trilogy by Timothy Zahn and the Dark Empire series by Tom Veitch were written in coordination with official Lucas Licensing continuity.

In the Dark Empire series, Emperor Palpatine is reborn into a clone body, and then when the clone is destroyed, reborn into another. However, this last clone is defective and begins degenerating quickly, with no other clone bodies available. By the end of the series, Palpatine is finally destroyed with no possibility of ever returning. When Lucas has been asked (possibly aware of the Dark Empire plotline, possibly not) whether Palpatine could have survived the events of Return of the Jedi, he has answered simply, "no, he's dead."

By contrast, Lucas also intended Boba Fett to die in Return of the Jedi when he fell into the Sarlacc pit, but he has accepted that character's return, which was also in Dark Empire. While on MTV (of all places) he indicated that he does not particularly care about this change, that fans and writers wanted to bring back Boba Fett, and so they did. He did not anticipate the popularity of the character, and while it surprised him, he is fine with Boba Fett's return.

He has made no such comment (to my knowledge) regarding the return of Emperor Palpatine. By contrast, he has made every indication that as far as he is concerned, Palpatine died in Return of the Jedi, never to return. According to the rules of continuity, this should mean the Dark Empire series never happened.

This is essentially impossible, as Dark Empire now holds an important place within the official Expanded Universe. New characters were introduced in the series, including the children of Han and Leia, who continue to be used in later stories. Trends begun in the Dark Empire series, such as the further development of new and existing super-weapons, continue in works by other writers. In many ways, the Thrawn trilogy and the Dark Empire series are a pivotal transition from the Expanded Universe stories surrounding and immediately following the original Star Wars trilogy, and the development of new eras in the saga's history.

So despite the contradiction with canon material, Palpatine does officially return in Dark Empire, and the entire series of comics really did occur. By extension, this also means the Sith returned six years after the Battle of Endor, only to be destroyed again within a matter of months.

However, the Sith had already returned once before, in stories that were both written earlier and placed earlier in the continuity. According to Marvel comics, Vader trained an apprentice under the nose of the Emperor, and passed on many Sith teachings to Lady Lumiya. When the Sith Order was destroyed at the Battle of Endor, Lumiya was training on the Sith homeworld, Ziost. She quickly took the title of Dark Lady of the Sith, and had several confrontations with Luke Skywalker. After the last fight between the only Jedi and the only Sith of the time, Lumiya escaped and has not appeared since.

The Lumiya storyline was outside of continuity for a number of years. When Lucas Licensing began coordinating an official continuity, problematic stories were simply ignored, and considered outside the events of "what really happens in Star Wars". Over the years, later writers have included more references to Lumiya, bringing her back into continuity - her apprentices, Flint and Carnor Jax, have reappeared and she was included in the article "The Emperor's Pawns". Then in 2004, the continuity rules changed, so that LFL-licensed sources all became official, and rather than contradictory stories being outside continuity, only those story elements that were contradictory are now considered inaccurate.

Therefore, Lumiya has once again been officially trained by Vader, and in the year following the Battle of Endor, she made several appearances, calling herself "Dark Lady of the Sith". However, because Lucas' sentiment that the Sith were destroyed at the Battle of Endor, the continuity experts (Leland Chee in particular) have determined that while Lumiya called herself Sith, she did not have the training and credentials to actually be Sith - or at least not at that time.

This is the kind of difficulty that currently exists with the Schrödinger-like rebirth of the Sith following Return of the Jedi. According to the canon sources, the Sith are destroyed and do not return. According to the official sources, not only do the Sith return, but Palpatine himself returns. However, unlike the return of Boba Fett, Lucas has given no indication that he has changed his mind regarding the death of the Sith in general or Palpatine in particular, at least until recently.

A new era has been announced for Star Wars, the Legacy era. In the announcement for the Legacy era, Sue Rostoni stated that Lumiya will return. This return will answer many questions, most importantly whether she is now truly Sith, or if the Sith were actually destroyed. The direction of the new era has been approved by Lucas, just as the New Jedi Order stories followed Lucas' direction for a non-Force-using group of villains. Therefore any return of the Sith in the Legacy era will indicate that the Sith can indeed return, but officially they do not do so for a generation after the death of Palpatine.

Personally I am hopeful, as Lucas has indicated he is stepping back further and further from Star Wars, and leaving its management in the hands of the experts at Lucas Licensing. The Sith will inevitably return every time they are destroyed.