Here's a few little retcons I've been tinkering with:
The scene: During Episode III, Mace Windu agrees to place Anakin on the Jedi Council, but does not give him the rank of Master.
Anakin fumes. He is outraged that he is placed on the Council but not given mastership. He says it's never been done in the history of the Jedi Order.
Meanwhile, someone off-screen glances at Ki-Adi-Mundi, thinking "wasn't he appointed as a Knight?"
Likely official story: Lucas did not know/care that Ki-Adi-Mundi was a Knight when he was appointed, having not read Star Wars Vol 2 #1-6 (later called Republic 1-6) or Jedi Council: Acts of War 1-4.
Retcon 1: The case of Ki-Adi-Mundi was that he was due to become a Master in a matter of weeks or even days, but the seat needed to be filled right away (on account of his predecessors' death). For all intents and purposes, Ki-Adi was a Master the whole time, but his Mastership ceremony was held after he was given his post.
Retcon 2 (which I like more): Anakin is an idiot. While Jedi training does include a certain amount of book education, Anakin was never a Temple Baby. He became a Padawan the instant he joined the Order, and thus missed out on nine years' worth of education that all other Jedi take for granted. While he was given the opportunity to continue learning whilst adventuring (hyperspace trips are long; he can read on the way), Anakin was never a book-smart kind of guy. He preferred lightsaber training and pilot training, and as a result fell behind in formal education. So when the time came for him to be appointed to the Jedi Order, there were a few key things that Anakin did not know. He did not know that Jedi could be appointed to the Council as Knights. He did not know that one Master, Ki-Adi-Mundi, was appointed as a Knight. He also did not know that, to become a Master, he needed to have trained at least one Padawan (usually more), and Ahsoka was still an Apprentice (I'm guessing; she's only 17 or so by ROTS. In the end, we don't know what happened to her, and I feel comfortable saying she's not a Knight in ROTS). So he didn't know that he was ineligible for Mastership anyway. Conclusion: Anakin was simply wrong in his "it's never been done" argument.
-Sparks Nevada, Marshal on Mars
"Who cares what evil lurks in the hearts of men!"
"Unless evil's carrying the Martini tray, darling."
-Frank and Sadie Doyle