Revealing mistake: During the scene when Hawkins is in the jousting tournament, Griswald takes a swing with his mace and apparently smashes Hawkins' head (inside his helmet) off. Then the horse Hawkins was riding stops, and he extends his head from inside his suit of armor and smiles. Watch carefully throughout this shot - Hawkins' hair/the back of his head can be seen past the edge of the armor where the hole for his neck should be.
Revealing mistake: Several shots and whole scenes throughout this film are flipped horizontally in such a blatant fashion that even the name of the ship, "Sans Souci," appears as a mirror image.