When Costa is in the bar, he receives a note instructing him "Meet me in the restroom". In Canada, the correct terminology is "washroom", and someone asking where the restroom is will frequently be met with blank looks.
During the car chase, there are several shots from the point of view of the chase car. The barrel of the gun being pointed at the driver in the lead car is clearly an older style revolver. However, in subsequent shots and flashbacks we see that the gun is a more modern semi-automatic.
While the movie is set in Montreal, Quebec, several scenes were filmed in Quebec City some 150 miles north east from Montreal. The bridge scene and Le chateau Frontenac are in Quebec City not Montreal. A police uniform is one scene has the Quebec City insignia.
When Scott is eating breakfast in the restaurant with the other detectives, a French-style police siren can be heard in the background. Canadian police sirens sound just like their American counterparts.
The police helicopters landing at the Moncton (New Brunswick) train station are Sûreté du Québec (Quebec provincial police) helicopters. Even if the SQ were authorized to carry out an intervention in New Brunswick, Moncton is over 200 kilometres from the Quebec border - there's no way those helicopters could have gotten there in the 20 minutes the police said they had.
On the way to Mont Laurier from St-Jovite, Matt tells Martin that he's going to Seattle. Matt would have had to go towards Montreal in order to go to Seattle by bus - Mont Laurier is in the opposite direction
One of the Montreal police officers reassures Costa that the serial killer "won't get within 50 yards of you". While Canada is officially metric, its proximity to the U.S. as well as stubbornness among older generations means that in several instances old imperial measurements are still used. Many people still use feet and inches instead of meters for height, and many Canadians still refer to yards instead of meters for distance.
When Ileana hears LeClair being shot, she races into the building with her sunglasses on. She has no sunglasses the whole time she is in the apartment, but has them back on as she races down the stairs to chase the other car.
In the final scene, Costa pulls a garrote from his back pocket and stretches the handles apart, making the ligature taut. At that point, the ligature is clearly wire, but by the time he wraps it around Illeana's throat, the ligature is thicker, appearing to be a string or cloth cord (most likely to avert possible injury to Ms. Jolie).
When the white Suburban is spiraling down the road after colliding with Matt (dummy) while changing the tire there is a crew member with a blue top and helmet and yellow pants behind a round hay bale in the field as the Suburban rolls by.
When Costa puts Hart's dead body in the car it clearly is secured with the seat belt. That would prevent the head from hitting the windshield at all in the first place, even more so to hit it so hard so as to actually break a hole big enough for the head to protrude.
When Agent Scott first drives Costa back to his apartment in the rain, she confirms the address, "219?". The shot then cuts to a building marked 188 and shows Hart standing in the entrance to this building, spying on Costa. We then see Costa entering a building, and the camera cuts back to Hart standing in the doorway of 188. While it is admittedly unclear, Costa does not enter building 188 and can be assumed by the viewer to be entering 219, since (as revealed later) a meeting with Hart would no doubt result in an altercation.