When the New Main Street Singers are playing at the reunion concert, members of the band move around between shots.
When Terry and Laurie Bohner are telling the story of how they met, Laurie's arm is alternately around Terry's shoulders/by her side between shots.
When he's playing alone in the motel room, one of Mitch Cohen's pill bottles moves between shots.
Two minutes into the movie, as Jonathan Steinbloom says he is an organized person, one of the brown packages on his desk is askew on the long shots, but straight in the close-ups.
When Mitch and Mickey are introducing their song at Town Hall, Mitch's hand is making a "C" chord. In the next shot with just Mickey's head and Mitch's guitar in the background, it has no hand on it. When the shot returns to Mitch and Mickey together, he is still making a "C" chord.
After Mitch and Mickey are introduced during the concert, there is an overhead distant shot of the stage. Mitch can be seen nodding to the guitarist to his left, and the two begin strumming - but no music is heard. Mickey also begins playing and her lips don't move, but she is heard thanking the audience. It then cuts to a two-shot, and nobody is playing while Mickey continues talking to the audience.
In the final Reunion Scene, you can obviously tell that none of the actors are actually playing guitar. Most of them just move their fingers like strumming, and you can clearly see no vibration from the strings or contact with the fingers and strings.
When Mr. Turpin of Town Hall (Michael Hitchcock) demonstrates the acoustics by butchering Ave Maria, he changes "dominus tecum" to "domunis tecum".