Deliberate mistake: When the Darlings discover the dead rat, they raise the curtain and cry out loud, but the rat is nowhere to be seen. Given Disney's long time history of refusals to show dead bodies or blood, it's highly possible that this was a deliberate mistake, but really awkward indeed.
Deliberate mistake: During the song Adelaide's Lament, Adelaide goes to her medicine cabinet, opens it, and there is a cut to a shot from "inside" the cabinet as she surveys all the medicine she takes to treat her "cold." From the inside, the cabinet is almost twice as wide as it is when it shows her open and later close it from the outside.