DCI Boyd repeatedly investigates alone. Scottish Police Procedures require two officers at all times for the sake of corroboration. By attending crime scenes, suspects, witnesses or other important lines of investigation alone, he would be ruining any evidence seen or heard, which would be disregarded by Scottish Courts due to lack of corroboration.
DCI Boyd continues working on the case even after his wife becomes a victim of the same murder spree. This would never be allowed and would result in any evidence gained from Boyd to be inadmissible in a court of law, shattering the case.
DCI Boyd attempts to question Macgregor at his front door. This is illegal, can not be carried out alone, can not be carried out away from a controlled Police Station and no suspect was ever given their rights prior to conversation attempts. Which is again illegal in Scotland and in breach of human rights.
DCI Boyd carries a firearm with him throughout the movie. This is contrary to firearms protocols. Scottish police do not routinely carry firearms.
When the Scottish detective crashes his BMW while chasing the murderer, the airbags do not deploy. Same for the automatic seat belt. This on a modern BMW? Yeah nah.
When Dan is leaning against his car in the ending scene, he is supposed to be standing in front of a mountain landscape but it is clearly a green screen.
Near the end of the movie where Boyd and Lawson are in the forest, Boyd's weapon keeps switching between a Sig Sauer and a Glock.
The film starts with a seven branch menorah in the window. This is never done. The 9 branch channukiah is indeed placed in the window at Channuka.
When Boyd is looking for his wife in his house, he dials her mobile phone and tracks the sound. When he discovers the phone it's inert and dark despite the incoming call. It's a small detail.
DCI Boyd calls over his radio that he is driving on Livingston Highway. Which doesn't exist.
DCI Boyd is supposed to be a Scottish Detective, yet touches key evidence in Lawsons hotel room without wearing forensic gloves. This is possibly intentional from the filmmakers to show how Boyds interest has become personal and no longer professional.
DCI Lawson has a firearm in his hotel room, which he discharges and kills a person . As a visiting USA police officer he would not be allowed to carry a firearm.
Neither, would he walk away from the shooting scene without being arrested.