When Stephanie goes to the office to get her cactus just before the end of the movie, her hair is flipped up in the back, but in the next shots, her hair is curled under in the back.
Toward the end of the movie, when Nurse Dickinson is straightening the waiting room and Señor Sanchez enters, she has on dark shoes. In a subsequent shot, she has on white shoes.
When Walter Matthau answers the phone in his office, no button lights up, so we know the phone isn't connected.
Before Toni turns on the gas, she raises the top of the stove and blows out the pilot lights. But she doesn't blow out the pilot light for the oven. She then turns on the oven and all the burners. The oven would have come on and Toni would have noticed, or if she hadn't noticed then there would have been an explosion.
The "cactus" on Stephanie Dickinson's desk appears to be a species of Euphorbia, and not actually a cactus plant. Euphorbias of this type have evolved physical characteristics and forms similar to cacti of North and South America, and they are often incorrectly referred to as cacti. The flowers bear no resemblance to the one in the movie.
When Julian is driving Stephanie home, the shot from the driver's side of the car reveals the shadow of the car against the traffic on the movie screen behind them.
When Julian is driving Stephanie home, something (possibly a boom mic) is reflected on the top center of background screen.
The letter Toni mails in the middle of the night to Julian would not have been delivered to his office later that same morning with the regular mail by the U.S. Postal Service - later the next day at the earliest is much more likely.