Giulietta's necklace is turned from jewels to wax by Dapertutto, however, in a longer shot, it is briefly shown as jewels again, before a close-up, where it is wax again until Dapertutto changes it back to jewels.
1 hour 36 minutes in, when Hoffmann is paying Antonia a visit, the position of Robert Rounseville's hands changes between shots.
15 minutes in, in the close-up of Lindorf as he spies on Hoffmann and the students, the edge of Robert Helpmann's fake scalp is visible under his hairpiece.
During Olympia's song, Moira Shearer isn't miming for quite lengthy sections - as she is pirouetting quite fast in some of these sections, it is understandable that she'd want to concentrate on her balance.