Around 38 minutes in, Catherine looks out of a diamond leaded window at Culpepper. We see her as the camera looks into the room, but looking outwards the shape of the lead framing is different. It is not just a question of focus as the soldering is different on the joints.
Also, at the end, both Henry and Culpepper look at Catherine in the rain and the device is not used.
Catherine Howard is introduced to Lady Elizabeth who looks to be a teenager. In reality when Catherine Howard was Queen Elizabeth was between 7-8.
Henry Howard says that Catherine Howard was his niece. However she was not. They were first cousins.
They mention a marriage between Mary and King Francis' son Henry. However Henry had already been married to Catherine De Medici for 7 years.
Henry Howard, Earl of Surrey refers to Catherine Howard as his niece. In reality, they were first cousins as grandchildren to the 2nd Duke of Norfolk. Additionally, despite the age gap in the characters, Surrey was only seven years older than Catherine.
Edward Seymore's wife has a modern hearing aid visible as she is dancing in the King's court.