Check the Cyrkle: the guy on double-necked guitar isn't even trying to look like he's playing. He seems more intent on cracking up the other singer.
Notice during the Academy Award Nominees song medley, the male dancer holding the slate for "The Umbrellas of Cherbourg" does not move out of the picture fast enough for Lesley Gore to start singing "I Will Wait For You."
During the Top Pop Medley, Jill Stuart forgets the words to "Houston" and eventually gets back on track.
During the Top Pop Medley, Cher forgets the words to "(I Can't Get No) Satisfaction" and Sonny points out where they are on the cue card to get her back on track.