Deliberate mistake: Foils, rapiers and other fencing weapons did not appear before around the 16th/17th century, yet the main characters (in an approximate 8th to 10th century setting) use them extensively. I know that back in the days when the movie was made, the swashbuckling genre was quite popular, but it still is a reality error.
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.
Deliberate mistake: Glenn Ford strikes the window in the library door with his briefcase and it instantly shatters. In reality, a blow delivered by a briefcase to a plate glass window would have produced no more than a scratch or crack.