In a scene before her trial, Rebecca is seen in her chamber praying. She is at the side of her bed on her knees. Jewish people do not kneel when they pray (except on Yom Kippur.)
When Sir Brian de Bois-Guilbert signs the letter, the words are in modern English, yet even at that time the Normans would have spoken and written in Norman French. If they despised the Saxons as inferior, there is no way they would condescend to even learn their language.
Just after Ivanhoe has defeated De Bracy and Bois Guilbert lines up to accept challenge, he is briefly seen holding De Bracy's shield instead of his own. In the next shot from the front, he once again holds his own shied.
At the very end as Rebecca leaves the castle, the sea is visible in the background. But York is inland.