Continuity: Jack's big rig has a huge Christmas wreath on the front grill. When the truck loses its brakes, there's an overhead view of it careening down the road, and the wreath has disappeared. It's back in the next shot, though.

Continuity: When Julie talks to Monk on the phone, her hair is down over her right shoulder but swept back behind her left one. When she hangs up and hands the phone to her mom, the hair is suddenly covering her left shoulder instead, and now her right shoulder is bare.

Plot hole: Monk's dad, Jack, drives an 18 wheeler. In the scene where Jack pulls over to the side of the mountain road so they can discuss specifics about the GPS, a brake line is shown breaking and spurting what I assume is meant to be brake fluid. The rig then rolls down the mountain. However, 18 wheelers don't have brake fluid since they don't have hydraulic brakes, they have air brakes. Even if the rig lost its brakes due to loss of air pressure, the brakes are spring loaded to lock up all the wheels in this case and prevent movement. That's how you set the parking brakes, by releasing the air pressure. In other words, if he has air, he can use his application brakes; if he's lost air, he has his parking brakes set. Rigs that lose their brakes going down steep grades do so due to extreme overheating, causing brake fade, an inability of the brake shoes to grab the brake drums. This wouldn't have happened while they were parked.

Visible crew/equipment: Jack opens the diner door for Monk, and its glass window panes mirror the camera crew and a big rectangular studio reflector.