Joe Mannix changes cars in the middle of the episode. He starts out driving the Cuda and then goes to the custom Dart from earlier seasons. Obviously, they used old footage to fill in some time.
When Peggy is typing the note, the bell rings while she's typing the word 'death,' according to the key sounds. Yet in the finished note, the words 'and death' are at the beginning of a new line.
Joe insists that Peggy be photographed holding each day's newspaper to prove she is unharmed. The first time that she does so, in the closeup the main headline reads "The 'Baby Bust'" in giant headlines. When the camera switches to a medium shot, however, the main headline is in smaller type, and reads, "Freeway Collision in Heavy Fog." Then the shot switches back to a closeup, and the newspaper again has the "Baby Bust" headline.
Just before the opening credits, notice the arrangement of the items on the desk after Peggy's typed letter is placed on it. When Joe returns, right after the opening credits, several items (ashtray, hole punch, etc.) have changed position or (vase) disappeared without anyone having had the opportunity to move them.
When Mannix is writing the note to Peggy he is obviously just scrawling across the paper. But the closeup shows him writing very legibly.
When Mannix shoots a guy off the pier around 39 minutes in, the background shows someone casually walking by in on the balcony of the house behind him. They don't react to the shots or the action on the pier.