When Mr. Henry Tilney meets Mrs. Morland, he virtually drains his glass, which is then refilled ("I should like to pay my respects"), and drained again ("Perhaps Miss Morland").
At one point the dance caller announces that the next dance will be "Upon a Summer's Day." This dance was actually performed in an earlier scene, and is not the next dance in sequence.
The background when Catherine reads the letter from her brother is very obviously fake. The camera sometimes shakes yet the background remains still as if it's been pasted over the original shot. It is most obvious at the 1:15:20 mark.
Catherine is shown multiple times reading, fantasizing, and talking about The Mysteries of Udolpho by Mrs. Radcliffe. And yet the bulk of her fantasies and voice over come from The Monk by Lewis, a much more lurid novel than the tame Mrs. Radcliffe's.