Okay, I've been up half the night reading Geoff Emerick's brilliant book and I come to page 152 and I quote:
Although the overdubs to the middle section were being done separately from the main body of the song, it had already been edited into the fourtrack master, which made Richard's job of dropping in and out a bit tricky. Paul's vocal, for example, was being dropped into the same track that contained John's lead vocal, and there was a very tight drop-out point between the two--between Paul's singing"...and I went into a dream" and John's "ahhh" that starts the next section. Richard was quite paranoid about it--with good reason--and I remember him asking me to get on the talkback mic to explain the situation to Paul and ask him not to deviate from the phrasing that he had used on the guide vocal. I was really impressed when Richard did that--I thought it showed great maturity to be proactive that way. John's vocal, after all, had such great emotion, and it also had tape echo on it. The thought of having to do it again and re-create the atmosphere was daunting...not to mention what John's reaction would have been! Someone's head would have been bitten off, and it most likely would have been mine. But Paul, ever professional, did heed the warning, and he made certain to end the last word distinctly in order to give Richard sufficient time to drop out before John's vocal came back in. Listening carefully, you can actually hear Paul slightly rush the vocal; he even adds a little "ah" to the end of the word "dream" giving it a very clipped ending.
(Even though I was wrong, I'm greatly relieved!)