Overused case in point: Qui Gon Jin
As had been explained to me on another forum, there is a difference between instructing and training. Therefore, when the ghost of Obi Wan refered to Yoda as "the Jedi master who instructed me," everyone watching in the theatre in 1981 thought to themselves "Oh, Yoda only instructed Obi Wan. I wonder who actually trained him."
Yeah right. And my butt occasionally plays that Macarena song.
No one thought that and to show Obi Wan being trained by a Jedi other than Yoda is jarring and contridictory. Worse, it was completely unnecessary. They could have easily just been friends and comrades in arms instead of a master and apprentice in any official capacity.
Lucas was always just making stuff up as he went along, but in the 15 or so years between ROTJ and TPM, he forgot what he was in doing that.