Audio problem: When Vader is talking to an Imperial officer in the hangar in the Death Star, when the officer says, "It must be a decoy, sir. Several of the escape pods have been jettisoned." his head is shown from behind, but his profile shows his mouth is moving before the words are heard. (01:03:55)
Audio problem: When C-3PO lowers himself into the oil tub, as Luke says, "It just isn't fair." his head is shown from the side and he then turns his head away from the camera, but the words don't match the motion of his jaw (or his lips, if you look closely). (00:19:00)
Audio problem: When the Millennium Falcon is attacked after the group escape from the Death Star, there is a TIE fighter which makes an X-Wing sound.
Audio problem: When Vader and Tarkin learn that Leia has lied about the location of the Rebel base, we hear Darth say "I told you she would never consciously betray the rebellion." There is a pause and Vader continues to move. An obvious dubbing error.
Audio problem: Widescreen version: When Tarkin attempts to make Leia reveal the location of the Rebel base, he says, "You would prefer another target? A military target? Then name the system." In the next shot, his head is shown from behind, but the motion of his jaw shows that he is still talking, even though nothing is heard. (00:56:00)
Audio problem: In the French version of the film, Han Solo tells Obi-Wan and Luke to go to hangar 49. 94 is written at the entrance.
Audio problem: When Luke, Han, Chewie, Ben and the droids are in the control room, and Luke says "but they're going to execute her," watch closely: his mouth doesn't move for half the sentence.
Audio problem: [Special Edition only] In the scene where Luke and Leia swings across the missing bridge, a number of the shots fired by Leia are "bangs" instead of the laser "zap" sound. The bang is probably from the prop gun used (it shot blanks) and was never dubbed over (until the 2004 DVD release).
Answer: It is heavily implied that one who becomes a Force-Ghost achieves an untold level of power upon entering the state. Given that they have become a pure entity of the Force, it seems to back up the statement. Obi-Wan also becomes free to assist Luke in any case.
Darius Angel
In addition to this answer, I think Obi-wan also became powerful because he "let go." He did not feel a need to defeat Vader on this occasion - he was willing to surrender the fight and "move on" to a new state. Something that Vader might not currently understand.