Notice that ANH and ESB never say who Anakin’s master was. They only say that Vader was Obi-Wan’s apprentice. It would be interesting if Yoda was Anakin’s master. In fact, in the Brackett draft of ESB, the Luke/Anakin lightsaber has the coordinates to Dagobah stored on it. This makes me think that Brackett intended for Yoda to be Anakin’s master. After Obi-Wan and Anakin complete their training, Obi-Wan, arrogant in his ability to be a master, trains Vader, and the rest is history. It would also be interesting if Yoda warned Obi-Wan, Anakin, and Vader not to get involved in the Clone Wars, but they ignore Yoda’s warning.