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 Hawkins mocks Ravenhurst by pouring himself some wine and then drinking it while fending off Ravenhurst's, Danny Kaye repeatedly uses the same fencing movements.