At around 52 minutes: When Jeff's nurse goes to the door saying she's going to find out the name of the freight carrier that is taking off with the trunk, someone who sounds nothing like James Stewart has dubbed him with the peculiar sounding line "I'll keep an eye on the alley". Jimmy Stewart is holding the binoculars over his mouth but we can see that he's not moving his lips! Prior to that, when he says "don't do anything foolish" his lips are still moving after the audio is heard.
When Miss Lonelyhearts and the songwriter are talking about his record in his apartment, the dubbed-in dialogue doesn't sync with the picture, even to the extent of Miss Lonelyhearts being heard to say "I can't tell you what this music has meant to me," while her mouth isn't moving.
While Lisa and Stella are digging up the flowers (about 92 minute mark) the pianist is shown playing with other musicians. When the harmonica players starts to play, a saxophone is heard although none is visible. The harmonica is subsequently heard also.
(at around 1h 18 mins) When Lisa and Jeff are discussing "rear window ethics", Grace Kelly noticeably stumbles her line "Jeff, you know if someone came in here they wouldn't believe what they'd s...see"
When Lisa places her slippers into her overnight case (while sitting on Jeff's lap), they are tossed in, more to the side of the case. Later, when the detective views the suitcase, the slippers are neatly placed and sitting upright.
Lisa takes the binoculars away from Jeff and wraps the neck cord around them before putting them on a small cupboard. When Jeff picks up the binoculars later, the neck cord is no longer wrapped around them.
When Lisa goes to Jeff's house to celebrate his last week with his cast, she places two candles onto the table next to him, the candles are not lit. A little later in the same scene, Lisa walks into the kitchen, and when she returns, the candles are lit.
When Thorwald returns home from one of his trips out in the rain lugging his suitcase, the camera (from Jeff's point of view) pans from a glimpse of Thorwald in the street, across Miss Torso's apartment where she is preparing to go to bed, to the second floor hallway where Thorwald is walking toward his apartment. This observed action takes only a few seconds - an impossibly short time frame for Thorwald to have entered his building through its front door, walked over to the stairwell, climbed the stairs to the second floor and then be seen walking along the second floor hallway.
When Lisa lowers Jeff's window shades for the evening, all of the shades are shown as being up at first. Lisa only lowers the shades on the front windows and the right-hand side window. She does not lower the left-hand window shade, and it remains up. However, when she comes out of the bathroom to model her nightgown for Jeff shortly afterwards, the camera angle now shows the left-hand shade being down too when no one was shown lowering it.
At the end of Jeff's first massage, Stella places the bottle with the green liquid on the side table without replacing the cap. As Stella is packing to leave, the bottle is capped as she places it in her bag.
The Exakta camera used in this movie is usually held in a way that would suggest that the shutter is triggered on the right-side upward surface, as is normal for most cameras. 35mm Exakta cameras actually have the shutter release on the front of the camera, just left of the lens.
(at about 1 hr 13 mins) Jeff wheels himself over to the window and bumps his leg (the one in the cast) against the wall below the window, yet he doesn't grimace at all. Once a broken leg in a cast begins healing, it's no more sensitive to such bumps than an uninjured leg.
As Lisa (played by Grace Kelly) is in the kitchen preparing the brandy for Lt. Thomas Doyle and Jeff, her silhouette can be seen on the ceiling. Her silhouette is of Grace Kelly though with her hair down. Though, moments later when she emerges with the warmed brandy, her hair is tied back in a chiffon.
The goof item below may give away important plot points.
The image retention effect depicted in Jeff's apartment, when he fires the flashbulbs to temporarily blind Thorwald, is based upon the color of the light source. The type of flashbulbs used were a Clear type, appearing bluish to the eye, not red as the POV shots indicate.