Continuity mistake: When Cinderella opens the curtain before waking up Lucifer, the hallway has a large window, a mirror and a stool beside the the stairs, but a door to music room has appeared on the hallway when Lucifer escapes the singing and playing of the stepsisters. This can be also noticed when stepmother hears Cinderella humming the ball waltz and goes after her, the window is back in its place rather than the door. The carpet is also covering whole floor in the scene where Lucifer exits the non-existing music room and the railing is shorter than in other scenes. (00:09:50 - 00:26:25)
Continuity mistake: When Cinderella ascends the stairs with three breakfast trays, Gus is under the cup in her right hand tray. That right hand tray (with Gus) is dropped off in the first of the three rooms in order, which is Drizella's room. When screams are eventually heard and Lucifer runs to catch Gus, they come from the second (middle) door, which is Anastasia's room and it's Anastasia who had the mouse.