The postmark on Ruby's letter to John's wife is one day earlier than the date of the assurance company's letter that John had received the day before, and six days earlier than the day of the month displayed on John's office wall calendar the day Ruby's letter was mailed.
Harmon says "goodbye" to the other person on the phone when he's put the handset almost back on the cradle, well away from his mouth.
When Mrs Harman receives the letter the address on the envelope is more to the left than it was when it was posted, most noticeable when compared to the left edge of the stamp.
Where does Jeffrey Hart get a key to open the case to try and steal a rare book? And, how is the window to the shop left unlocked for Jeffrey to get in?