(at around 44 mins) The license plate on Atticus Finch's car changes. At first it is 709-865 as Atticus backs his car out of the garage for his first visit to Helen Robinson and again later as he approaches the garage for the second trip. The plate is not seen during the first visit at the Robinson house, but when he reaches there the second time (at around 1h 47 mins) , it is 369-883.
(at around 16 mins) When sitting on the porch with the Judge, Atticus is sitting on the porch swing with his left arm on the arm-rest. However, during the solo shots of the judge, it is obvious that no one is sitting on the swing - there is no arm on the arm-rest and the swing's chains are loose.
Mayella Ewell's hair alternates between neatly brushed and messy and back again between shots when she gets up to go to the stand in the courtroom.
(at around 1h 29 mins) When Mr. Gilmer gets up to cross examine Tom Robinson, it cuts to a shot of Atticus at the defense table. But the person who walks past the camera at that point is wearing a very anachronistic black t-shirt revealing the wearer's bare arms, instead of Mr Gilmer's pale-colored suit.
(at around 1h 2 mins) When the rednecks from Old Sarum arrive at the jail alleyway to lynch Tom Robinson, the rear shot of them approaching Atticus shows a very young lout in the group holding a rifle (in a light shirt with sleeves rolled to his biceps and no hat), but in all the following shots from the front, he's nowhere to be found.
Tom Robinson cannot use his left arm because the muscles were injured in an accident when he was a child. In such a case, atrophy would have rendered that arm thin and wasted. Both of Tom's arms are equally and impressively muscular.
During Atticus Finch's cross-examination of Mayella Ewell, he breaks off, has defendant Tom Robinson stand and catch an object with his right hand as a demonstration, and asks him to explain (not under oath) why he can't catch the object with his left hand; Atticus then resumes his cross-examination of Mayella. In a real trial, a lawyer can't examine two witnesses simultaneously; if he tried to do so, the opposing lawyer would successfully object to it. Moreover, if a defendant were to testify, as Tom Robinson effectively did (even if from the defense table, not under oath), the prosecutor would object, and could insist on cross-examining the defendant under oath.
When Atticus shoots the dog with the high-powered rifle, the gun does not recoil (kick).
Atticus stated, "Until you climb inside of his skin and walk around in it." Later Scout misquotes her father, saying, "One time Atticus said you never really knew a man until you stood in his shoes and walked around in them."
However, there's no reason why Scout couldn't have paraphrased her father's intended message in her own words.
(at around 2h 8 mins) The law books disappear from the mantelpiece when Scout returns from walking Boo Radley home. This would be the bedroom with a fireplace and a woman's photograph on its mantel. The law books were never on the mantel, they were on a dresser (or tall writing desk) beside the bed (at around 1h 58 mins). In some shots, they seemed to be on a mantel (at 2h 1 min) because the dresser is cropped off by the bottom of the frame.
(at around 13 mins) After Atticus speaks with Mrs. Dubose at her porch, Scout, Jem, and Dill walk away with Atticus towards the Finch home while the camera pauses on Mrs. Dubose pondering Atticus' words. When the camera cuts back to the Finches, they have walked over to their house, up their sidewalk, and are at the steps to their porch while Dill has gone past their sidewalk, past their driveway and is standing by the rope swing in their side yard looking back at the Finches as if he were watching them arrive home. It looks awkward, as if it were taken from another scene where they were arriving home, but it could have happened that way if Dill were too shy to accompany them up their sidewalk without being invited (which, given Dill's personality, doesn't seem likely).
At the end of the movie, Scout mentions the gifts Boo Radley has given them over the years, including "a broken watch and chain". However, during the opening credits, the watch face looks intact and the watch is ticking loudly. (The watch has no hands; fake hands have been drawn on the face of the watch. Hence, the watch is broken). The watch is also visible and ticking softly when Jem first shows Scout his box of things he found in the knothole of the tree in front of the Radley house.
(at around 48 mins) When Atticus carries the sleeping Scout down the hall to her bedroom, after returning from their visit to the Robinson's, he takes her all the way to the last room on the left, which is actually Jem's room. Actually, it is a hallway. Calpurnia is holding the door to the house open when Atticus reaches the porch, revealing a door with a 15-pane French window at the end of the entryway. Atticus carries Scout all the way to this door and turns left, not into a bedroom but into a hallway to the children's rooms. Their bedrooms were shown when Scout looks at Atticus' watch: the camera is pointing from Scout's window, across Scout's bed at the solid door to the dining room in the center of the frame, just beyond Scout's table lamp (at around 13 mins), with the dining room visible in the background on the right side of the frame and the wall behind Scout's headboard along the left side of the frame. After kissing Scout goodnight and turning her lamp out, Atticus swings the dining room door from left to right revealing the solid door to Jem's bedroom which he enters walking to the left, tells Jem goodnight, shuts off Jem's light, comes out, closes Jem's door, and walks down a hall to the right, which would be the hall into which he turned carrying Scout.
(at around 30 mins) When Scout and Jem are debating Jem going back to retrieve his trousers from Boo Radley's, Scout can be seen mouthing Jem's lines.
Directly after the scene where Jem and Scout are attacked while walking home through the woods, as Scout runs after the figure carrying Jem home, the trees and scenery can be seen through Scout in a ghostly fashion as if they were not originally part of the scene and were added afterward. Director Robert Mulligan mentions in the DVD commentary that this is the only special effect in the movie (at around 1h 56 mins). This was necessitated because the extended shot shows the transition of Boo and Scout from the woods to the Finch house, because everything was shot at Universal Studios (Universal City, California), and because there were no woods near the studio re-creation of Maycomb.
(at around 44 mins) Similar to her "close-up" conversation with Jem about retrieving his breeches, Scout mouths Atticus' next lines when begging to be allowed a ride to visit Tom Robinson's family before the court case. The lip movement is slight and would be mumbling were she speaking, but when Atticus says, "Promise to stay in the car while I talk with Helen Robinson" the movement does seem to match "Helen Robinson".
(at around 43 mins) When Atticus shoots the mad dog, it is obvious its hind legs are jerked from under it to make it fall.
(at around 55 mins) In a closeup of Mr. Radley troweling grout into the knothole after Scout and Jem have run off, someone coughs, but it is not Mr. Radley, whose chin can be seen.
At the beginning, a penny in the cigar box dated 1960 is shown in the lower right corner of the screen, while the story is set in the year 1932.
The "melon" Crayola crayon shown in the opening sequence was not introduced until 1949. The film is set in 1932.
The PF Flyers worn by Jem and Scout were not introduced until 1937.
The Crayola crayons shown in the opening (and elsewhere in the film) with the black bands were introduced in 1948. The film is set in the 1930s.
During the opening credits, when a marble is rolling, the camera and members of the camera crew are reflected in the marble it strikes just before "Introducing" appears on the screen.
When Tom Robinson is driven past the Finch home, Atticus tells Jem that he is being brought back to Maycomb from the Abbottsville jail, but the car is driving away from town.
In the background when Atticus kills the "mad" dog is a large hill, or mountain. No such geography exists in the Maycomb Al area.
(at around 1h 12 mins) When Bob Ewell testifies he says he saw Tom Robinson running out the front door. When Mayella Ewell testifies, she says when she came to, her father was standing over her yelling who done it? Both stories can't be true, but neither Atticus Finch nor anybody else points this out.
When Jem and Scout leave the tree with the knothole in it when Mr. Radley is filling in the knothole, he leaves his school books at the foot of the tree. At 0:53:59, Jem drops his books by the tree to adjust Scout's feet to "walk like an Egyptian". At 0:55:02 they run off without the books.
In the very beginning of the film, the adult Scout narrates, "Maycomb was a tired, old town, even in 1932 when I first knew it." Moments later, she goes on to say that "Maycomb County had recently been told that it had nothing to fear but fear itself" in clear reference to Franklin D. Roosevelt's first inaugural address. Roosevelt, however, did not deliver this speech until March 4, 1933.
Atticus defines a compromise as "an agreement reached by mutual consent". All agreements are reached by mutual consent. A compromise is specifically reached by mutual concessions.
(at around 11 mins) When the children are shown waiting for Atticus to come home, Jem is lying in the tire of the rope swing counting each time the swing changes directions while the courthouse clock chimes seven times. As the clock is chiming for the sixth time, Jem slips out of the tire and starts running down the sidewalk yelling, "Come on Scout! It's five o'clock."
In the final scene, the narrator says "He (Atticus) would be in Jem's room all night and he would be there when Jem waked up in the morning." Atticus may have stayed all night in the room where Jem was sleeping, but it wasn't Jem's bedroom; it was probably Atticus' bedroom considering all of the law books about the room.