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: 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.