The fact that Lizzie Talvo finds a road where she can gain such a high speed before she hit the brakes the first time is almost impossible. Most people's first step for getting in a car is to depress the brake. If a car is in a driveway it's likely that the person will brake in the driveway. It's unlikely anyone would get on a road where you could travel at such a high speed on a winding road with out braking. The brake line should have been cut enough where she would have noticed a problem or it gave out before then.
The Car crash occurred at 7:39 pm on May 2 in the Los Angeles area and the crash was a night scene. In early May in the Los Angeles area this would be less than 5 minutes after sunset, and it would still be nearly full daylight and certainly not a black sky at this time.
Monk says the police didn't check out Lizzie Talvo's death because it wasn't a criminal investigation. Her car went off a road for no reason, and even a casual check would have revealed that the brake line was cut rather than frayed. There would have been an investigation.
When Monk finds the discarded video tape cassette he makes a point of keeping it on the assumption it may be important to the case. However, he makes no effort to keep the envelope it was in, which might itself have provided some much needed information for the case (e.g., fingerprints).
The answering machine message that Monk hears at the house of Val Birch says that the Mercedes "640" has arrived. There is no Mercedes Benz 640, nor has there ever been one. There is however a BMW 640, so it is possible that the dealership also carries the BMW brand and Monk wrongly assumed that it was a new Merc that Val had ordered.
When Monk first goes upstairs with his mother-in-law, he hesitates when he wants to look in at his late wife's bedroom. The bedroom has a door that opens into the room and swings to the right. At the end of the story, he garners the courage to enter, this time the door opens in but swings to the left.
On several of the questions, Val and Monk both ring in and their buzzers can be heard simultaneously. However, Jeopardy! and similar game shows invariably have "lockout"-type buzzer systems that prevent more than one buzzer signaling at a time. These systems also prevent a person from ringing in before the question has been completely asked. Also, all game shows have a time limit on how long you can take to answer questions (in Jeopardy! for instance, it's five to seven seconds).
Lizzie drives a late model Audi. Like all modern cars, Audis have two separate hydraulic brake circuits. Cutting one brake line would only disable one of them leaving the car with enough stopping power to at least slow down. Cutting a rear brake line, as Roddy did, would have the least effect, since approximately 70% of braking is done by the front brakes.