When Danny and Sonny come to the auditorium, three female singers take the stage in the 1940's sequence. All 3 mouth the words, but but Olivia Newton-John does all of the singing. One dark-haired singer occasionally forgets to move her lips.
When Sonny discovers Kira skating on the floor of the Xanadu building, he calls "You!" from the balcony above. Kira's repartee is "Me," which she says as her face is set towards the ground. Immediately after, she looks up at Sonny and can be seen silently mouthing the same line.
In the opening shots of the "All Over the World" sequence, outside the entrance to the clothing boutique, the mannequin furthest from the camera shouts out counts to keep the dancers together. The only audio heard at that point is the prerecorded song, but his lips are very clearly moving.
During the "Dancin'" scene, as the three singers start walking down the stairs, one of the Zoot Suit dancers is on his knees playing with his hat directly in front of them. When the camera pulls back for the next shot he is to the right with the sailor between him and the singers.
When Sonny first sees the auditorium that eventually becomes Xanadu, the letters on the roof of the building that spell out auditorium are missing the A and R. Later when Kira takes him to the auditorium, the T is missing. In the following scene, when Sonny and Danny arrive at the auditorium, the T is back. Finally, when Sonny and Kira are slow dancing in front of the auditorium before it transforms into the finished Xanadu exterior, the T is missing again.
At the beginning of the movie, the muses in the painting on the mural are detailed and fully rendered. Later, when Sonny is about to jump into the mural, the muses in the painting are sketchy and lack detail.
In "The Fall" musical sequence, Sonny roller skates alone along a beach-side boardwalk. In the opening shot, heat waves shimmer up from the ground, and passersby are dressed in bathing suits and other warm-weather clothing. In subsequent shots the sky is grey and the people in the background are wearing heavy winter coats.
When Sonny first roller skates to the auditorium, the "No Trespassing" sign has the name of then-LA County Supervisor Edmund B. Edelman on it. When Sonny skates across the sign, the camera cuts to a zoomed-in shot in which the bottom quarter of the sign, including Edelman's name, is covered by what appears to be a white piece of paper.
In the wide shot of Sonny arriving at the auditorium, and in a closer shot when Sonny is at the doors, the "Warning! No Trespassing" sign clearly has the logo of the company who owns the building near the bottom. In all other shots the name is covered by some kind of tape or something.
Near the end of "Dancin'", when the two 80's dancers meet the two 40's dancers, a white scrim is behind them. Several pairs of white sneakers and blue jean-clad legs are visible behind it, at the bottom.
When Danny and Sonny meet, everyone around them is wearing warm weather clothing, like bikinis and short shorts. Danny and Sonny are both wearing long-sleeved coats, and cold-weather pants. Danny is wearing a wool scarf and a hat.
In the auditorium where Sonny and Kira first talk, Sonny says "You". Kira replies, "Me." Immediately after that line as Olivia Newton-John is skating out of frame at the bottom, she makes a silly face at the camera.
When Sonny chases after ONJ shouting "I don't have a very good reason.." he passes through shadows (to the right of the screen, his left) and light which don't land on him (you can see them through him).