At :15 into the film, Cary Grant's character, whose name has been seen in numerous photos identified as "Johnnie Aysgarth" (and is so-named in the closing credits), a telegram to Joan Fontaine's character is signed, "Johnny."
While playing "Anagrams" and Lina changes the letters MUDDER to MURDER she slides the D above the word and slides the R in it's place. The D is the only up turned tile directly above the word MURDER. After the camera switches to a close up of Lina and back the the tiles there are now and H and F up turned and 2 face down tiles next to them that weren't there seconds ago.
Lina pulls a letter from an envelope which is shorter than the book that contains it. But in the next shot it is evident that the paper she now holds is wider than the length of book below. The close up of the handwritten note indicates it was written on a much narrower page size
When Lina looks at Isobel's book while Johnnie is on the telephone, there is a large tear in the book jacket above Isobel's photo. In the next scene, when Lina takes the book on her visit to Isobel, the book jacket is undamaged.
In the dinner scene at her parent's house after Lina's walk with Johnny, the glasses of water in front of her parents are both full, but during the course of their conversation with Lina, neither of them take a drink of water. When Lina leaves to answer the phone, but glasses are empty.
Alfred Hitchcock makes his cameo appearance about forty-six minutes in, posting a letter in the village. Note the background in this shot and a shot just a few seconds earlier with the car driving by and a man walking while smoking. They are both exactly the same background shot with same car, people, with one being enlarged. It could be the film was so expensive, the re-use of background shots was needed.
In the final scene as Johnnie and Lina drive back home, it is clear from the shot of them from behind that doubles have been used and the two characters in the scene are not Joan Fontaine and Cary Grant.