As Lori is driving, she is carrying on a conversation with John, but the passenger window is clearly rolled up (reflections are visible). When he opens the car door in the reverse shot, it is rolled down.
When we see the shot of "Action News Georgia," the backdrop is of Atlanta. However, damage to the Westin Peachtree Plaza is visible. This damage is from a tornado that hit downtown Atlanta in 2008, well after the news report from 1985.
When Ted is bear-napped by Donny and Robert, and he escapes into the bathroom to call John for help, he calls from a land-line phone. Discovering him, Donny ends the call by disconnecting the telephone wire from the wall, yet we hear a dial tone. A land-line phone would go completely silent once its cord became disconnected.
When Ted is escaping from Donny's house, Donny is dancing in front of the TV while sipping a soda. The camera switches to show Ted running by the door, and suddenly Donny is dancing with his hands on his hips, and there's no glass of soda to be seen.
When Ted and John sit in the park and attempt to smoke pot, the bench is right next to the tree trunk. After they are approached by Donny and his son, they are shown again sitting on the same bench that is now several feet to the left of the tree trunk.
After crashing into the Volvo the Jetta's hood is pushed in. But when the Jetta comes to a stop in front of the hijacker's house the hood is fine apart from the license plate hanging loose and some dirt on the grill.
When Rex goes to talk to Lori at her office desk, the shot shows him with his hair combed back, next shot, his hair is a different style with more camera light. In the next shot it's back to the original combed back look with the darker camera lighting.
During the car chase, they are seen driving from Boston proper through the Ted Williams Tunnel. Shortly afterward, they arrive at Fenway Park. To do this, they would have had to drive back through the tunnel.
The supermarket where Ted works is supposed to be in Boston, but the reusable tote bags for sale at the register are from "Best Yet Market", which is only in the New York City area. It's a different store name than what is on the cashiers' name tags as well.
When John and Lori arrive at Donny's house in Chelsea to rescue Ted, they are driving the wrong way on Chestnut Street (a one-way street). Donny leaves, with Lori & John following, going the wrong way on Pine St. (also a one-way street).
Near the end of the movie, the characters are able to sneak into Fenway Park with little difficulty due to a garage door being partially left open. This obviously would not be the case in real life. Also, with no baseball game going on and no cleaning crew visible, the stadium lights would not be left on for no reason.
Lori manages to find John at Ted's home warming party but John didn't tell her the address. Likewise Ted finds the hotel John was staying at after Lori dumped him despite John not telling him where he was.
During John and Lori's anniversary dinner at the restaurant, after John says "Let's just enjoy our anniversary dinner", the view of the street outside is clearly waving, as if it were a printed on a canvas and made to look like the outside of the restaurant.