Tom's truck is a GMC when he is working on getting it running. However, when driving to Philly, you can see Chevrolet is written on the steering wheel.
The bathroom door alternates between open and closed after Edie emerges in her cheerleader outfit.
When Sam the Sheriff comes to the house for the second time (after the second batch of killings), he parks diagonally at the beginning of the driveway, essentially blocking the entrance to the driveway. He and Tom/Joey go into the house to talk, and while they are talking, Edie pulls into the garage, parks and comes into the house. She could not have pulled into the garage with the Sheriff's car blocking the driveway.
When Sam (the Sheriff) stops Fogarty's car and talks to him, he looks at the driver's license in his hand. Immediately afterward, Fogarty closes the window, and the car drives away, without Sam returning the license. Yet his hands are empty.
While Tom is in the hospital after the shootout at the diner, his wife shows him the front page of the news paper with him on it. Not possible for the newspaper to write/print and deliver the paper that night before his wife makes it to the hospital.
When Carl Fogarty's black Chrysler pulls into the mall parking lot, the required Pennsylvania inspection tags are not on the bottom of the driver's side of the windshield.
The signboard for the strip plaza advertises a Home Hardware Store. This is a Canadian hardware chain.
Richie's back is to the camera when Joey shoots him, and the muzzle flash from the pistol is shown. At that short range the bullet strike should be simultaneous with the muzzle flash, but it isn't shown until the point of view moves behind Joey.
The shot of Tom/Joey entering the Track & Turf bar in Philly, shows several neon signs of beer logos. The Beck's one faces into the room. But being mounted in the window it would be expected to be turned around so it could be read from outside.
The Toy Store in the mall/plaza has a sign in the window indicating that Chargex credit cards are accepted. Chargex was a uniquely Canadian credit card that was used from 1968 to 1977 (prior to Visa).
In the high school softball game, the bully comes up to the plate carrying a wooden bat, but the sound heard when he hits the ball is the sound of an aluminum bat.
When the high school kids are playing softball you can hear
the ball hitting the bat earlier than the player strikes.
When Edie drives up to the shopping center, the shadow of the camera and crane is visible in the driver window as she opens the door.
Although quite out of focus and in the distance, two police cruisers can easily be seen blocking traffic with their overhead lights on as Tom is driving towards Philly.
When William Orser drives the car in front of the office at the beginning of the movie, the boom and some crewmembers are reflected in the front fender.
When the camera cranes down on the black car as it's being pulled over, the camera is reflected in the driver's window.
There is a Canadian speed limit sign visible when the main character is driving to Philadelphia.
When Tom is driving from Indiana to Philadelphia, a sign in the highway lists the travel distances as "Indianapolis 36", "Columbus 247" and "Philadelphia 680". But Indianapolis in 170 miles west of Columbus and 585 miles from Philadelphia, so if he is 36 miles from Indianapolis, he's only 206 miles from Columbus and 611 miles from Philadelphia.
(at around 13 mins) When Carl Fogarty first shows up at Stall's Diner, he tells Tom/Joey that his name is 'Joey Dewsack' (mispronouncing 'Cusack' twice). It is most pronounced the second time he says it.