On David and Leslie's drive, David is driving and Leslie is in the front passenger seat just before David turns on the radio. This segment is shot in close-up. In the next long shot, there is no one sitting in the front passenger seat.
When David and Leslie almost kiss and are interrupted by a phone call, David says, "we have to get to Canada". As David is getting into the drivers side of the car, you can see the reflection of the Canadian Maple Leaf from the Tim Hortons sign on the back passenger window. They are already in Canada.
When David and Leslie are in Frankenmurth, MI, there is a business sign with a telephone number with the area code 705, which is not for anywhere in Michigan but rather a region in Ontario (where the movie was filmed).
When David and Libby are in Frankenmuth, all the vehicles have a front license plates. Michigan does not have, provide, or require a front plate.