When Caul is in Stett's office alone, he walks over to the desk and picks up one of Stett's wife's cookies. He smells it and puts it back in the dish and then looks through the telescope. When Stett returns, he hands Caul the money and takes the tapes. When the film cuts to a shot of Caul thinking about the arrangement, the cookie reappears. Caul puts this cookie back in the dish, too.
Throughout the film, Harry listens to the recording of Mark telling Ann,"He'd kill us if he got the chance," which creates empathy in Harry and motivates his actions in the story. However, at the end of the film, after the Director has been killed, the recording plays again, and a new emphasis completely changes the meaning of the playback: "He'd kill US if he had the chance," indicating that Mark and Ann are justifying their plan to kill the Director, not the other way around. There is no explanation for the change in the recording.
When Harry Caul is in the elevator, you see the woman come in and three buttons are pushed. After stopping on a floor, a cut back to the shot of the woman and only one button remains lit whereas two should still be lit.
When Harry is talking with Bernie in the workshop during the party, the chain on the device hanging from the ceiling is swaying vigorously, but when the scene cuts mid-sentence, the chain is now perfectly still.
Harry Caul is the top surveillance expert with custom equipment that he designed himself and is extremely paranoid but illogically allows the other surveillance experts to enter his lab and examine his equipment.