On Abbey Road, during "I Want You (She's So Heavy)," right after John screams "YEEAAHH!!" at 4:27, you can hear someone in the studio shouting, "CUT THAT OUT JOHN!" Also during another song (I forget which), you can hear someone momentarily counting along with the music.
What have you heard?
I might as well start intentionally listening for this stuff, it might be as fun as it was when I was looking through the "Where's Waldo?" books as a child.