(at around 44 mins) When the killer saws off one of the woman's arms in the elevator, blood sprays onto her face. The blood from the victim's face disappears and reappears between shots.
(at around 44 mins) Before one of the victims' gets one of her arms sawed off, the woman's left arm that she moves changes position - which is a dummy arm facing upwards - in two shots.
The puzzle in the beginning changes inconsistently in a couple shots; complete, partial on one side and then nearly complete. Also, the kid still is still able to put pieces of the puzzle on when in the previous shot, it was clearly finished.
(at around 2 mins) After Timmy kills the mother in the prologue, her mouth is open when she dies. In the next shot, it is closed; this also happens in a flashback.
(at around 1h 2 mins) While Kendall goes to investigate something suspicious in the locker room, he sets aside his tennis supplies on the bench; albeit when Mary and Willard return, the stuff has disappeared.
(at around 4 min) At the beginning of the film after the mother is killed, the neighbor that comes to visit with the police is asked about the father's whereabouts and mentions him being in the Air Force. Because the prologue takes place in 1942, the U.S. Air Force didn't exist until a good five years later and that the main U.S. Flying Force was called the U.S. Army Air Corps until the end of World War II.
The jigsaw puzzle used has blood still fresh and red after forty years of wearing out; albeit around the time the film takes place, wouldn't the blood be brown?
(at around 56 mins) As the killer stabs the fourth victim through the back of her head, the "prop" knife clearly bends the other way.
(at around 36 mins) When the tennis pro is playing tennis, it is obvious that her skill level is that of a beginner.
(at around 44 mins) When the killer saws off the victim's arms in the elevator, it's obvious that the saw chains on the chainsaw are not moving.
A shot of the Dean's footsteps are recycled twice in some scenes.
Before the fifth victim is killed, it's quite obvious that the door has been sculpted with clay on one spot where the chainsaw comes in through from previous takes.
When the police officers arrive to investigate Timmy's mother's murder, they are both dubbed by the same actor.
(at around 1 min) Although the prologue is set in 1942, the mother tells the boy to get a plastic trash bag, even though it was invented in 1950. Aside from the American version that has this goof, the Spanish version she just asks for a dustbin.
(at around 4 mins) Though the film begins in the 1940's, a pennant for the New England Patriots can be easily viewed on the wall of a young boy. The Patriots didn't come into existence until 1960, and even then they were originally known as the Boston Patriots.
(at around 2 mins) During the prologue set in 1942, a 1970's khaki push-button telephone is shown facing from behind. Around that time, the phones would've been colored black, vintage and they would be rotary.
(at around 7 mins) When the killer is digging through the mother's remains, he finds a photograph of her laying on her side in the grass; the picture taken is obviously too trendy from it being from about the nineteen-forties.
(at around 8 mins) After the girl on a skateboard crashes into a glass mirror and it breaks into pieces, the camera's reflection can be seen briefly (with a green tarp covering one side) as the mirror shatters.
In the present day, when the Dean removes a box from the drawer - he finds his mother's bloodied dress and shoes. If he has them, how was he able to take them away as evidence from the police back forty years ago?
As the Dean asks him "Wouldn't you say?" Willard answers early "That's for sure" speaking his response over the Dean's line before the question has even been asked.
After he's turned down by Francine, the dubbing leaves the first word "Who" off Alister's line "gives a s--t".